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diff --git a/1228-0.txt b/1228-0.txt new file mode 100644 index 0000000..026988f --- /dev/null +++ b/1228-0.txt @@ -0,0 +1,16196 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 1228 *** + +There are several editions of this ebook in the Project Gutenberg collection. +Various characteristics of each ebook are listed to aid in selecting the +preferred file. + +Click on any of the filenumbers below to quickly view each ebook. + +1228 1859, First Edition +22764 1860, Second Edition +2009 1872, Sixth Edition, considered the definitive edition. + + + + +On +the Origin of Species + +BY MEANS OF NATURAL SELECTION, + +OR THE +PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE. + +By Charles Darwin, M.A., + +Fellow Of The Royal, Geological, Linnæan, Etc., Societies; +Author Of ‘Journal Of Researches During H.M.S. Beagle’s Voyage +Round The World.’ + +LONDON: +JOHN MURRAY, ALBEMARLE STREET. +1859. + +“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.” + +W. WHEWELL: _Bridgewater Treatise_. + +“To conclude, therefore, let no man out of a weak conceit of sobriety, or +an ill-applied moderation, think or maintain, that a man can search too far or +be too well studied in the book of God’s word, or in the book of +God’s works; divinity or philosophy; but rather let men endeavour an +endless progress or proficience in both.” + +BACON: _Advancement of Learning_. + +_Down, Bromley, Kent, + October_, 1_st_, 1859. + + +Contents + + INTRODUCTION. + 1. VARIATION UNDER DOMESTICATION. + 2. VARIATION UNDER NATURE. + 3. STRUGGLE FOR EXISTENCE. + 4. NATURAL SELECTION. + 5. LAWS OF VARIATION. + 6. DIFFICULTIES ON THEORY. + 7. INSTINCT. + 8. HYBRIDISM. + 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + 11. GEOGRAPHICAL DISTRIBUTION. + 12. GEOGRAPHICAL DISTRIBUTION—_continued_. + 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: + 14. RECAPITULATION AND CONCLUSION. + INDEX + +DETEAILED CONTENTS. ON THE ORIGIN OF SPECIES. + +INTRODUCTION. + + + + +CHAPTER I. VARIATION UNDER DOMESTICATION. + + Causes of Variability. + Effects of Habit. + Correlation of Growth. + Inheritance. + Character of Domestic Varieties. + Difficulty of distinguishing between Varieties and Species. + Origin of Domestic Varieties from one or more Species. + Domestic Pigeons, their Differences and Origin. + Principle of Selection anciently followed, its Effects. + Methodical and Unconscious Selection. + Unknown Origin of our Domestic Productions. + Circumstances favourable to Man’s power of Selection. + +CHAPTER 2. VARIATION UNDER NATURE. + + Variability. + Individual Differences. + Doubtful species. + Wide ranging, much diffused, and common species vary most. + Species of the larger genera in any country vary more than the + species of the smaller genera. + Many of the species of the larger genera resemble varieties in being + very closely, but unequally, related to each other, and in having + restricted ranges. + +CHAPTER 3. STRUGGLE FOR EXISTENCE. + + Bears on natural selection. + The term used in a wide sense. + Geometrical powers of increase. + Rapid increase of naturalised animals and plants. + Nature of the checks to increase. + Competition universal. + Effects of climate. + Protection from the number of individuals. + Complex relations of all animals and plants throughout nature. + Struggle for life most severe between individuals and varieties of + the same species; often severe between species of the same genus. + The relation of organism to organism the most important of all + relations. + +CHAPTER 4. NATURAL SELECTION. + + Natural Selection: its power compared with man’s selection, its power + on characters of trifling importance, its power at all ages and on + both sexes. + Sexual Selection. + On the generality of intercrosses between individuals of the same + species. + Circumstances favourable and unfavourable to Natural Selection, + namely, intercrossing, isolation, number of individuals. + Slow action. + Extinction caused by Natural Selection. + Divergence of Character, related to the diversity of inhabitants of + any small area, and to naturalisation. + Action of Natural Selection, through Divergence of Character and + Extinction, on the descendants from a common parent. + Explains the Grouping of all organic beings. + +CHAPTER 5. LAWS OF VARIATION. + + Effects of external conditions. + Use and disuse, combined with natural selection; organs of flight and + of vision. + Acclimatisation. + Correlation of growth. + Compensation and economy of growth. + False correlations. + Multiple, rudimentary, and lowly organised structures variable. + Parts developed in an unusual manner are highly variable: specific + characters more variable than generic: secondary sexual characters + variable. + Species of the same genus vary in an analogous manner. + Reversions to long-lost characters. + Summary. + +CHAPTER 6. DIFFICULTIES ON THEORY. + + Difficulties on the theory of descent with modification. + Transitions. + Absence or rarity of transitional varieties. + Transitions in habits of life. + Diversified habits in the same species. + Species with habits widely different from those of their allies. + Organs of extreme perfection. + Means of transition. + Cases of difficulty. + Natura non facit saltum. + Organs of small importance. + Organs not in all cases absolutely perfect. + The law of Unity of Type and of the Conditions of Existence embraced + by the theory of Natural Selection. + +CHAPTER 7. INSTINCT. + + Instincts comparable with habits, but different in their origin. + Instincts graduated. + Aphides and ants. + Instincts variable. + Domestic instincts, their origin. + Natural instincts of the cuckoo, ostrich, and parasitic bees. + Slave-making ants. + Hive-bee, its cell-making instinct. + Difficulties on the theory of the Natural Selection of instincts. + Neuter or sterile insects. + Summary. + +CHAPTER 8. HYBRIDISM. + + Distinction between the sterility of first crosses and of hybrids. + Sterility various in degree, not universal, affected by close + interbreeding, removed by domestication. + Laws governing the sterility of hybrids. + Sterility not a special endowment, but incidental on other + differences. + Causes of the sterility of first crosses and of hybrids. + Parallelism between the effects of changed conditions of life and + crossing. + Fertility of varieties when crossed and of their mongrel offspring + not universal. + Hybrids and mongrels compared independently of their fertility. + Summary. + +CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + + On the absence of intermediate varieties at the present day. + On the nature of extinct intermediate varieties; on their number. + On the vast lapse of time, as inferred from the rate of deposition + and of denudation. + On the poorness of our palæontological collections. + On the intermittence of geological formations. + On the absence of intermediate varieties in any one formation. + On the sudden appearance of groups of species. + On their sudden appearance in the lowest known fossiliferous strata. + +CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + + On the slow and successive appearance of new species. + On their different rates of change. + Species once lost do not reappear. + Groups of species follow the same general rules in their appearance + and disappearance as do single species. + On Extinction. + On simultaneous changes in the forms of life throughout the world. + On the affinities of extinct species to each other and to living + species. + On the state of development of ancient forms. + On the succession of the same types within the same areas. + Summary of preceding and present chapters. + +CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. + + Present distribution cannot be accounted for by differences in + physical conditions. + Importance of barriers. + Affinity of the productions of the same continent. + Centres of creation. + Means of dispersal, by changes of climate and of the level of the + land, and by occasional means. + Dispersal during the Glacial period co-extensive with the world. + +CHAPTER 12. GEOGRAPHICAL DISTRIBUTION—_continued_. + + Distribution of fresh-water productions. + On the inhabitants of oceanic islands. + Absence of Batrachians and of terrestrial Mammals. + On the relation of the inhabitants of islands to those of the nearest + mainland. + On colonisation from the nearest source with subsequent modification. + Summary of the last and present chapters. + +CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: +EMBRYOLOGY: RUDIMENTARY ORGANS. + + CLASSIFICATION, groups subordinate to groups. + Natural system. + Rules and difficulties in classification, explained on the theory of + descent with modification. + Classification of varieties. + Descent always used in classification. + Analogical or adaptive characters. + Affinities, general, complex and radiating. + Extinction separates and defines groups. + MORPHOLOGY, between members of the same class, between parts of the + same individual. + EMBRYOLOGY, laws of, explained by variations not supervening at an + early age, and being inherited at a corresponding age. + RUDIMENTARY ORGANS; their origin explained. + Summary. + +CHAPTER 14. RECAPITULATION AND CONCLUSION. + + Recapitulation of the difficulties on the theory of Natural + Selection. + Recapitulation of the general and special circumstances in its + favour. + Causes of the general belief in the immutability of species. + How far the theory of natural selection may be extended. + Effects of its adoption on the study of Natural history. + Concluding remarks. + +ON THE ORIGIN OF SPECIES. + + + + +INTRODUCTION. + + +When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with +certain facts in the distribution of the inhabitants of South America, +and in the geological relations of the present to the past inhabitants +of that continent. These facts seemed to me to throw some light on the +origin of species—that mystery of mysteries, as it has been called by +one of our greatest philosophers. On my return home, it occurred to me, +in 1837, that something might perhaps be made out on this question by +patiently accumulating and reflecting on all sorts of facts which could +possibly have any bearing on it. After five years’ work I allowed +myself to speculate on the subject, and drew up some short notes; these +I enlarged in 1844 into a sketch of the conclusions, which then seemed +to me probable: from that period to the present day I have steadily +pursued the same object. I hope that I may be excused for entering on +these personal details, as I give them to show that I have not been +hasty in coming to a decision. + +My work is now nearly finished; but as it will take me two or three +more years to complete it, and as my health is far from strong, I have +been urged to publish this Abstract. I have more especially been +induced to do this, as Mr. Wallace, who is now studying the +natural history of the Malay archipelago, has arrived at almost exactly +the same general conclusions that I have on the origin of species. Last +year he sent to me a memoir on this subject, with a request that I +would forward it to Sir Charles Lyell, who sent it to the Linnean +Society, and it is published in the third volume of the Journal of that +Society. Sir C. Lyell and Dr. Hooker, who both knew of my work—the +latter having read my sketch of 1844—honoured me by thinking it +advisable to publish, with Mr. Wallace’s excellent memoir, some brief +extracts from my manuscripts. + +This Abstract, which I now publish, must necessarily be imperfect. I +cannot here give references and authorities for my several statements; +and I must trust to the reader reposing some confidence in my accuracy. +No doubt errors will have crept in, though I hope I have always been +cautious in trusting to good authorities alone. I can here give only +the general conclusions at which I have arrived, with a few facts in +illustration, but which, I hope, in most cases will suffice. No one can +feel more sensible than I do of the necessity of hereafter publishing +in detail all the facts, with references, on which my conclusions have +been grounded; and I hope in a future work to do this. For I am well +aware that scarcely a single point is discussed in this volume on which +facts cannot be adduced, often apparently leading to conclusions +directly opposite to those at which I have arrived. A fair result can +be obtained only by fully stating and balancing the facts and arguments +on both sides of each question; and this cannot possibly be here done. + +I much regret that want of space prevents my having the satisfaction of +acknowledging the generous assistance which I have received from very +many naturalists, some of them personally unknown to me. I cannot, +however, +let this opportunity pass without expressing my deep obligations to Dr. +Hooker, who for the last fifteen years has aided me in every possible +way by his large stores of knowledge and his excellent judgment. + +In considering the Origin of Species, it is quite conceivable that a +naturalist, reflecting on the mutual affinities of organic beings, on +their embryological relations, their geographical distribution, +geological succession, and other such facts, might come to the +conclusion that each species had not been independently created, but +had descended, like varieties, from other species. Nevertheless, such a +conclusion, even if well founded, would be unsatisfactory, until it +could be shown how the innumerable species inhabiting this world have +been modified, so as to acquire that perfection of structure and +coadaptation which most justly excites our admiration. Naturalists +continually refer to external conditions, such as climate, food, etc., +as the only possible cause of variation. In one very limited sense, as +we shall hereafter see, this may be true; but it is preposterous to +attribute to mere external conditions, the structure, for instance, of +the woodpecker, with its feet, tail, beak, and tongue, so admirably +adapted to catch insects under the bark of trees. In the case of the +misseltoe, which draws its nourishment from certain trees, which has +seeds that must be transported by certain birds, and which has flowers +with separate sexes absolutely requiring the agency of certain insects +to bring pollen from one flower to the other, it is equally +preposterous to account for the structure of this parasite, with its +relations to several distinct organic beings, by the effects of +external conditions, or of habit, or of the volition of the plant +itself. + +The author of the ‘Vestiges of Creation’ would, I presume, say that, +after a certain unknown number of +generations, some bird had given birth to a woodpecker, and some plant +to the misseltoe, and that these had been produced perfect as we now +see them; but this assumption seems to me to be no explanation, for it +leaves the case of the coadaptations of organic beings to each other +and to their physical conditions of life, untouched and unexplained. + +It is, therefore, of the highest importance to gain a clear insight +into the means of modification and coadaptation. At the commencement of +my observations it seemed to me probable that a careful study of +domesticated animals and of cultivated plants would offer the best +chance of making out this obscure problem. Nor have I been +disappointed; in this and in all other perplexing cases I have +invariably found that our knowledge, imperfect though it be, of +variation under domestication, afforded the best and safest clue. I may +venture to express my conviction of the high value of such studies, +although they have been very commonly neglected by naturalists. + +From these considerations, I shall devote the first chapter of this +Abstract to Variation under Domestication. We shall thus see that a +large amount of hereditary modification is at least possible, and, what +is equally or more important, we shall see how great is the power of +man in accumulating by his Selection successive slight variations. I +will then pass on to the variability of species in a state of nature; +but I shall, unfortunately, be compelled to treat this subject far too +briefly, as it can be treated properly only by giving long catalogues +of facts. We shall, however, be enabled to discuss what circumstances +are most favourable to variation. In the next chapter the Struggle for +Existence amongst all organic beings throughout the world, which +inevitably follows from their high geometrical powers of +increase, will be treated of. This is the doctrine of Malthus, applied +to the whole animal and vegetable kingdoms. As many more individuals of +each species are born than can possibly survive; and as, consequently, +there is a frequently recurring struggle for existence, it follows that +any being, if it vary however slightly in any manner profitable to +itself, under the complex and sometimes varying conditions of life, +will have a better chance of surviving, and thus be _naturally +selected_. From the strong principle of inheritance, any selected +variety will tend to propagate its new and modified form. + +This fundamental subject of Natural Selection will be treated at some +length in the fourth chapter; and we shall then see how Natural +Selection almost inevitably causes much Extinction of the less improved +forms of life and induces what I have called Divergence of Character. +In the next chapter I shall discuss the complex and little known laws +of variation and of correlation of growth. In the four succeeding +chapters, the most apparent and gravest difficulties on the theory will +be given: namely, first, the difficulties of transitions, or in +understanding how a simple being or a simple organ can be changed and +perfected into a highly developed being or elaborately constructed +organ; secondly the subject of Instinct, or the mental powers of +animals, thirdly, Hybridism, or the infertility of species and the +fertility of varieties when intercrossed; and fourthly, the +imperfection of the Geological Record. In the next chapter I shall +consider the geological succession of organic beings throughout time; +in the eleventh and twelfth, their geographical distribution throughout +space; in the thirteenth, their classification or mutual affinities, +both when mature and in an embryonic condition. In the last chapter I +shall give a +brief recapitulation of the whole work, and a few concluding remarks. + +No one ought to feel surprise at much remaining as yet unexplained in +regard to the origin of species and varieties, if he makes due +allowance for our profound ignorance in regard to the mutual relations +of all the beings which live around us. Who can explain why one species +ranges widely and is very numerous, and why another allied species has +a narrow range and is rare? Yet these relations are of the highest +importance, for they determine the present welfare, and, as I believe, +the future success and modification of every inhabitant of this world. +Still less do we know of the mutual relations of the innumerable +inhabitants of the world during the many past geological epochs in its +history. Although much remains obscure, and will long remain obscure, I +can entertain no doubt, after the most deliberate study and +dispassionate judgment of which I am capable, that the view which most +naturalists entertain, and which I formerly entertained—namely, that +each species has been independently created—is erroneous. I am fully +convinced that species are not immutable; but that those belonging to +what are called the same genera are lineal descendants of some other +and generally extinct species, in the same manner as the acknowledged +varieties of any one species are the descendants of that species. +Furthermore, I am convinced that Natural Selection has been the main +but not exclusive means of modification. + + + + +CHAPTER I. +VARIATION UNDER DOMESTICATION. + + +Causes of Variability. Effects of Habit. Correlation of Growth. +Inheritance. Character of Domestic Varieties. Difficulty of +distinguishing between Varieties and Species. Origin of Domestic +Varieties from one or more Species. Domestic Pigeons, their Differences +and Origin. Principle of Selection anciently followed, its Effects. +Methodical and Unconscious Selection. Unknown Origin of our Domestic +Productions. Circumstances favourable to Man’s power of Selection. + + +When we look to the individuals of the same variety or sub-variety of +our older cultivated plants and animals, one of the first points which +strikes us, is, that they generally differ much more from each other, +than do the individuals of any one species or variety in a state of +nature. When we reflect on the vast diversity of the plants and animals +which have been cultivated, and which have varied during all ages under +the most different climates and treatment, I think we are driven to +conclude that this greater variability is simply due to our domestic +productions having been raised under conditions of life not so uniform +as, and somewhat different from, those to which the parent-species have +been exposed under nature. There is, also, I think, some probability in +the view propounded by Andrew Knight, that this variability may be +partly connected with excess of food. It seems pretty clear that +organic beings must be exposed during several generations to the new +conditions of life to cause any appreciable amount of variation; and +that when the organisation has once begun to vary, it generally +continues to vary for many generations. +No case is on record of a variable being ceasing to be variable under +cultivation. Our oldest cultivated plants, such as wheat, still often +yield new varieties: our oldest domesticated animals are still capable +of rapid improvement or modification. + +It has been disputed at what period of life the causes of variability, +whatever they may be, generally act; whether during the early or late +period of development of the embryo, or at the instant of conception. +Geoffroy St. Hilaire’s experiments show that unnatural treatment of the +embryo causes monstrosities; and monstrosities cannot be separated by +any clear line of distinction from mere variations. But I am strongly +inclined to suspect that the most frequent cause of variability may be +attributed to the male and female reproductive elements having been +affected prior to the act of conception. Several reasons make me +believe in this; but the chief one is the remarkable effect which +confinement or cultivation has on the functions of the reproductive +system; this system appearing to be far more susceptible than any other +part of the organisation, to the action of any change in the conditions +of life. Nothing is more easy than to tame an animal, and few things +more difficult than to get it to breed freely under confinement, even +in the many cases when the male and female unite. How many animals +there are which will not breed, though living long under not very close +confinement in their native country! This is generally attributed to +vitiated instincts; but how many cultivated plants display the utmost +vigour, and yet rarely or never seed! In some few such cases it has +been found out that very trifling changes, such as a little more or +less water at some particular period of growth, will determine whether +or not the plant sets a seed. I cannot here enter on the copious +details which I have collected on +this curious subject; but to show how singular the laws are which +determine the reproduction of animals under confinement, I may just +mention that carnivorous animals, even from the tropics, breed in this +country pretty freely under confinement, with the exception of the +plantigrades or bear family; whereas, carnivorous birds, with the +rarest exceptions, hardly ever lay fertile eggs. Many exotic plants +have pollen utterly worthless, in the same exact condition as in the +most sterile hybrids. When, on the one hand, we see domesticated +animals and plants, though often weak and sickly, yet breeding quite +freely under confinement; and when, on the other hand, we see +individuals, though taken young from a state of nature, perfectly +tamed, long-lived, and healthy (of which I could give numerous +instances), yet having their reproductive system so seriously affected +by unperceived causes as to fail in acting, we need not be surprised at +this system, when it does act under confinement, acting not quite +regularly, and producing offspring not perfectly like their parents or +variable. + +Sterility has been said to be the bane of horticulture; but on this +view we owe variability to the same cause which produces sterility; and +variability is the source of all the choicest productions of the +garden. I may add, that as some organisms will breed most freely under +the most unnatural conditions (for instance, the rabbit and ferret kept +in hutches), showing that their reproductive system has not been thus +affected; so will some animals and plants withstand domestication or +cultivation, and vary very slightly—perhaps hardly more than in a state +of nature. + +A long list could easily be given of “sporting plants;” by this term +gardeners mean a single bud or offset, which suddenly assumes a new and +sometimes very different character from that of the rest of the plant. +Such buds can be propagated by grafting, etc., and sometimes by seed. +These “sports” are extremely rare under nature, but far from rare under +cultivation; and in this case we see that the treatment of the parent +has affected a bud or offset, and not the ovules or pollen. But it is +the opinion of most physiologists that there is no essential difference +between a bud and an ovule in their earliest stages of formation; so +that, in fact, “sports” support my view, that variability may be +largely attributed to the ovules or pollen, or to both, having been +affected by the treatment of the parent prior to the act of conception. +These cases anyhow show that variation is not necessarily connected, as +some authors have supposed, with the act of generation. + +Seedlings from the same fruit, and the young of the same litter, +sometimes differ considerably from each other, though both the young +and the parents, as Müller has remarked, have apparently been exposed +to exactly the same conditions of life; and this shows how unimportant +the direct effects of the conditions of life are in comparison with the +laws of reproduction, and of growth, and of inheritance; for had the +action of the conditions been direct, if any of the young had varied, +all would probably have varied in the same manner. To judge how much, +in the case of any variation, we should attribute to the direct action +of heat, moisture, light, food, etc., is most difficult: my impression +is, that with animals such agencies have produced very little direct +effect, though apparently more in the case of plants. Under this point +of view, Mr. Buckman’s recent experiments on plants seem extremely +valuable. When all or nearly all the individuals exposed to certain +conditions are affected in the same way, the change at first appears to +be directly due to such conditions; but in some cases it can be shown +that quite opposite conditions produce +similar changes of structure. Nevertheless some slight amount of change +may, I think, be attributed to the direct action of the conditions of +life—as, in some cases, increased size from amount of food, colour from +particular kinds of food and from light, and perhaps the thickness of +fur from climate. + +Habit also has a decided influence, as in the period of flowering with +plants when transported from one climate to another. In animals it has +a more marked effect; for instance, I find in the domestic duck that +the bones of the wing weigh less and the bones of the leg more, in +proportion to the whole skeleton, than do the same bones in the +wild-duck; and I presume that this change may be safely attributed to +the domestic duck flying much less, and walking more, than its wild +parent. The great and inherited development of the udders in cows and +goats in countries where they are habitually milked, in comparison with +the state of these organs in other countries, is another instance of +the effect of use. Not a single domestic animal can be named which has +not in some country drooping ears; and the view suggested by some +authors, that the drooping is due to the disuse of the muscles of the +ear, from the animals not being much alarmed by danger, seems probable. + +There are many laws regulating variation, some few of which can be +dimly seen, and will be hereafter briefly mentioned. I will here only +allude to what may be called correlation of growth. Any change in the +embryo or larva will almost certainly entail changes in the mature +animal. In monstrosities, the correlations between quite distinct parts +are very curious; and many instances are given in Isidore Geoffroy St. +Hilaire’s great work on this subject. Breeders believe that long limbs +are almost always accompanied by an elongated head. Some instances of +correlation are quite whimsical; thus +cats with blue eyes are invariably deaf; colour and constitutional +peculiarities go together, of which many remarkable cases could be +given amongst animals and plants. From the facts collected by +Heusinger, it appears that white sheep and pigs are differently +affected from coloured individuals by certain vegetable poisons. +Hairless dogs have imperfect teeth; long-haired and coarse-haired +animals are apt to have, as is asserted, long or many horns; pigeons +with feathered feet have skin between their outer toes; pigeons with +short beaks have small feet, and those with long beaks large feet. +Hence, if man goes on selecting, and thus augmenting, any peculiarity, +he will almost certainly unconsciously modify other parts of the +structure, owing to the mysterious laws of the correlation of growth. + +The result of the various, quite unknown, or dimly seen laws of +variation is infinitely complex and diversified. It is well worth while +carefully to study the several treatises published on some of our old +cultivated plants, as on the hyacinth, potato, even the dahlia, etc.; +and it is really surprising to note the endless points in structure and +constitution in which the varieties and sub-varieties differ slightly +from each other. The whole organisation seems to have become plastic, +and tends to depart in some small degree from that of the parental +type. + +Any variation which is not inherited is unimportant for us. But the +number and diversity of inheritable deviations of structure, both those +of slight and those of considerable physiological importance, is +endless. Dr. Prosper Lucas’s treatise, in two large volumes, is the +fullest and the best on this subject. No breeder doubts how strong is +the tendency to inheritance: like produces like is his fundamental +belief: doubts have been thrown on this principle by theoretical +writers alone. When a +deviation appears not unfrequently, and we see it in the father and +child, we cannot tell whether it may not be due to the same original +cause acting on both; but when amongst individuals, apparently exposed +to the same conditions, any very rare deviation, due to some +extraordinary combination of circumstances, appears in the parent—say, +once amongst several million individuals—and it reappears in the child, +the mere doctrine of chances almost compels us to attribute its +reappearance to inheritance. Every one must have heard of cases of +albinism, prickly skin, hairy bodies, etc., appearing in several +members of the same family. If strange and rare deviations of structure +are truly inherited, less strange and commoner deviations may be freely +admitted to be inheritable. Perhaps the correct way of viewing the +whole subject, would be, to look at the inheritance of every character +whatever as the rule, and non-inheritance as the anomaly. + +The laws governing inheritance are quite unknown; no one can say why +the same peculiarity in different individuals of the same species, and +in individuals of different species, is sometimes inherited and +sometimes not so; why the child often reverts in certain characters to +its grandfather or grandmother or other much more remote ancestor; why +a peculiarity is often transmitted from one sex to both sexes or to one +sex alone, more commonly but not exclusively to the like sex. It is a +fact of some little importance to us, that peculiarities appearing in +the males of our domestic breeds are often transmitted either +exclusively, or in a much greater degree, to males alone. A much more +important rule, which I think may be trusted, is that, at whatever +period of life a peculiarity first appears, it tends to appear in the +offspring at a corresponding age, though sometimes earlier. In many +cases this could +not be otherwise: thus the inherited peculiarities in the horns of +cattle could appear only in the offspring when nearly mature; +peculiarities in the silkworm are known to appear at the corresponding +caterpillar or cocoon stage. But hereditary diseases and some other +facts make me believe that the rule has a wider extension, and that +when there is no apparent reason why a peculiarity should appear at any +particular age, yet that it does tend to appear in the offspring at the +same period at which it first appeared in the parent. I believe this +rule to be of the highest importance in explaining the laws of +embryology. These remarks are of course confined to the first +_appearance_ of the peculiarity, and not to its primary cause, which +may have acted on the ovules or male element; in nearly the same manner +as in the crossed offspring from a short-horned cow by a long-horned +bull, the greater length of horn, though appearing late in life, is +clearly due to the male element. + +Having alluded to the subject of reversion, I may here refer to a +statement often made by naturalists—namely, that our domestic +varieties, when run wild, gradually but certainly revert in character +to their aboriginal stocks. Hence it has been argued that no deductions +can be drawn from domestic races to species in a state of nature. I +have in vain endeavoured to discover on what decisive facts the above +statement has so often and so boldly been made. There would be great +difficulty in proving its truth: we may safely conclude that very many +of the most strongly-marked domestic varieties could not possibly live +in a wild state. In many cases we do not know what the aboriginal stock +was, and so could not tell whether or not nearly perfect reversion had +ensued. It would be quite necessary, in order to prevent the effects of +intercrossing, that only a +single variety should be turned loose in its new home. Nevertheless, as +our varieties certainly do occasionally revert in some of their +characters to ancestral forms, it seems to me not improbable, that if +we could succeed in naturalising, or were to cultivate, during many +generations, the several races, for instance, of the cabbage, in very +poor soil (in which case, however, some effect would have to be +attributed to the direct action of the poor soil), that they would to a +large extent, or even wholly, revert to the wild aboriginal stock. +Whether or not the experiment would succeed, is not of great importance +for our line of argument; for by the experiment itself the conditions +of life are changed. If it could be shown that our domestic varieties +manifested a strong tendency to reversion,—that is, to lose their +acquired characters, whilst kept under unchanged conditions, and whilst +kept in a considerable body, so that free intercrossing might check, by +blending together, any slight deviations of structure, in such case, I +grant that we could deduce nothing from domestic varieties in regard to +species. But there is not a shadow of evidence in favour of this view: +to assert that we could not breed our cart and race-horses, long and +short-horned cattle, and poultry of various breeds, and esculent +vegetables, for an almost infinite number of generations, would be +opposed to all experience. I may add, that when under nature the +conditions of life do change, variations and reversions of character +probably do occur; but natural selection, as will hereafter be +explained, will determine how far the new characters thus arising shall +be preserved. + +When we look to the hereditary varieties or races of our domestic +animals and plants, and compare them with species closely allied +together, we generally perceive in each domestic race, as already +remarked, less uniformity of character than in true species. Domestic +races of +the same species, also, often have a somewhat monstrous character; by +which I mean, that, although differing from each other, and from the +other species of the same genus, in several trifling respects, they +often differ in an extreme degree in some one part, both when compared +one with another, and more especially when compared with all the +species in nature to which they are nearest allied. With these +exceptions (and with that of the perfect fertility of varieties when +crossed,—a subject hereafter to be discussed), domestic races of the +same species differ from each other in the same manner as, only in most +cases in a lesser degree than, do closely-allied species of the same +genus in a state of nature. I think this must be admitted, when we find +that there are hardly any domestic races, either amongst animals or +plants, which have not been ranked by some competent judges as mere +varieties, and by other competent judges as the descendants of +aboriginally distinct species. If any marked distinction existed +between domestic races and species, this source of doubt could not so +perpetually recur. It has often been stated that domestic races do not +differ from each other in characters of generic value. I think it could +be shown that this statement is hardly correct; but naturalists differ +most widely in determining what characters are of generic value; all +such valuations being at present empirical. Moreover, on the view of +the origin of genera which I shall presently give, we have no right to +expect often to meet with generic differences in our domesticated +productions. + +When we attempt to estimate the amount of structural difference between +the domestic races of the same species, we are soon involved in doubt, +from not knowing whether they have descended from one or several +parent-species. This point, if it could be cleared up, would be +interesting; if, for instance, it could be shown that the greyhound, +bloodhound, terrier, spaniel, and bull-dog, which we all know propagate +their kind so truly, were the offspring of any single species, then +such facts would have great weight in making us doubt about the +immutability of the many very closely allied and natural species—for +instance, of the many foxes—inhabiting different quarters of the world. +I do not believe, as we shall presently see, that all our dogs have +descended from any one wild species; but, in the case of some other +domestic races, there is presumptive, or even strong, evidence in +favour of this view. + +It has often been assumed that man has chosen for domestication animals +and plants having an extraordinary inherent tendency to vary, and +likewise to withstand diverse climates. I do not dispute that these +capacities have added largely to the value of most of our domesticated +productions; but how could a savage possibly know, when he first tamed +an animal, whether it would vary in succeeding generations, and whether +it would endure other climates? Has the little variability of the ass +or guinea-fowl, or the small power of endurance of warmth by the +rein-deer, or of cold by the common camel, prevented their +domestication? I cannot doubt that if other animals and plants, equal +in number to our domesticated productions, and belonging to equally +diverse classes and countries, were taken from a state of nature, and +could be made to breed for an equal number of generations under +domestication, they would vary on an average as largely as the parent +species of our existing domesticated productions have varied. + +In the case of most of our anciently domesticated animals and plants, I +do not think it is possible to come to any definite conclusion, whether +they have descended from one or several 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 records, +more especially on the monuments of Egypt, much diversity in the +breeds; and that some of the breeds closely resemble, perhaps are +identical with, those still existing. Even if this latter fact were +found more strictly and generally true than seems to me to be the case, +what does it show, but that some of our breeds originated there, four +or five thousand years ago? But Mr. Horner’s researches have rendered +it in some degree probable that man sufficiently civilized to have +manufactured pottery existed in the valley of the Nile thirteen or +fourteen thousand years ago; and who will pretend to say how long +before these ancient periods, savages, like those of Tierra del Fuego +or Australia, who possess a semi-domestic dog, may not have existed in +Egypt? + +The whole subject must, I think, remain vague; nevertheless, I may, +without here entering on any details, state that, from geographical and +other considerations, I think it highly probable that our domestic dogs +have descended from several wild species. In regard to sheep and goats +I can form no opinion. I should think, from facts communicated to me by +Mr. Blyth, on the habits, voice, and constitution, etc., of the humped +Indian cattle, that these had descended from a different aboriginal +stock from our European cattle; and several competent judges believe +that these latter have had more than one wild parent. With respect to +horses, from reasons which I cannot give here, I am doubtfully inclined +to believe, in opposition to several authors, that all the races have +descended from one wild stock. Mr. Blyth, whose opinion, from his large +and varied stores of knowledge, I should value more than that of almost +any one, thinks that all the breeds of poultry have proceeded from the +common wild +Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the +breeds of which differ considerably from each other in structure, I do +not doubt that they all have descended from the common wild duck and +rabbit. + +The doctrine of the origin of our several domestic races from several +aboriginal stocks, has been carried to an absurd extreme by some +authors. They believe that every race which breeds true, let the +distinctive characters be ever so slight, has had its wild prototype. +At this rate there must have existed at least a score of species of +wild cattle, as many sheep, and several goats in Europe alone, and +several even within Great Britain. One author believes that there +formerly existed in Great Britain eleven wild species of sheep peculiar +to it! When we bear in mind that Britain has now hardly one peculiar +mammal, and France but few distinct from those of Germany and +conversely, and so with Hungary, Spain, etc., but that each of these +kingdoms possesses several peculiar breeds of cattle, sheep, etc., we +must admit that many domestic breeds have originated in Europe; for +whence could they have been derived, as these several countries do not +possess a number of peculiar species as distinct parent-stocks? So it +is in India. Even in the case of the domestic dogs of the whole world, +which I fully admit have probably descended from several wild species, +I cannot doubt that there has been an immense amount of inherited +variation. Who can believe that animals closely resembling the Italian +greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.—so +unlike all wild Canidæ—ever existed freely in a state of nature? It has +often been loosely said that all our races of dogs have been produced +by the crossing of a few aboriginal species; but by crossing we can get +only forms in some degree intermediate between their parents; and if we +account for our several domestic races by this process, we must admit +the former existence of the most extreme forms, as the Italian +greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover, the +possibility of making distinct races by crossing has been greatly +exaggerated. There can be no doubt that a race may be modified by +occasional crosses, if aided by the careful selection of those +individual mongrels, which present any desired character; but that a +race could be obtained nearly intermediate between two extremely +different races or species, I can hardly believe. Sir J. Sebright +expressly experimentised for this object, and failed. The offspring +from the first cross between two pure breeds is tolerably and sometimes +(as I have found with pigeons) extremely uniform, and everything seems +simple enough; but when these mongrels are crossed one with another for +several generations, hardly two of them will be alike, and then the +extreme difficulty, or rather utter hopelessness, of the task becomes +apparent. Certainly, a breed intermediate between _two very distinct_ +breeds could not be got without extreme care and long-continued +selection; nor can I find a single case on record of a permanent race +having been thus formed. + +_On the Breeds of the Domestic Pigeon_.—Believing that it is always +best to study some special group, I have, after deliberation, taken up +domestic pigeons. I have kept every breed which I could purchase or +obtain, and have been most kindly favoured with skins from several +quarters of the world, more especially by the Honourable W. Elliot from +India, and by the Honourable 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 greatly +elongated eyelids, very large external orifices to the nostrils, and a +wide gape of mouth. The short-faced tumbler has a beak in outline +almost like that of a finch; and the common tumbler has the singular +and strictly inherited habit of flying at a great height in a compact +flock, and tumbling in the air head over heels. The runt is a bird of +great size, with long, massive beak and large feet; some of the +sub-breeds of runts have very long necks, others very long wings and +tails, others singularly short tails. The barb is allied to the +carrier, but, instead of a very long beak, has a very short and very +broad one. The pouter has a much elongated body, wings, and legs; and +its enormously developed crop, which it glories in inflating, may well +excite astonishment and even laughter. The turbit has a very short and +conical beak, with a line of reversed feathers down the breast; and it +has the habit of continually expanding slightly the upper part of the +oesophagus. The Jacobin has the feathers so much reversed along the +back of the neck that they form a hood, and it has, proportionally to +its size, much elongated wing and tail feathers. The trumpeter and +laugher, as their names express, utter a very different coo from the +other breeds. The fantail has thirty or even forty tail-feathers, +instead of twelve or fourteen, the normal number in all members of the +great pigeon family; and these feathers are kept expanded, and are +carried so erect that in good birds the head and tail +touch; the oil-gland is quite aborted. Several other less distinct +breeds might have been specified. + +In the skeletons of the several breeds, the development of the bones of +the face in length and breadth and curvature differs enormously. The +shape, as well as the breadth and length of the ramus of the lower jaw, +varies in a highly remarkable manner. The number of the caudal and +sacral vertebræ vary; as does the number of the ribs, together with +their relative breadth and the presence of processes. The size and +shape of the apertures in the sternum are highly variable; so is the +degree of divergence and relative size of the two arms of the furcula. +The proportional width of the gape of mouth, the proportional length of +the eyelids, of the orifice of the nostrils, of the tongue (not always +in strict correlation with the length of beak), the size of the crop +and of the upper part of the oesophagus; the development and abortion +of the oil-gland; the number of the primary wing and caudal feathers; +the relative length of wing and tail to each other and to the body; the +relative length of leg and of the feet; the number of scutellæ on the +toes, the development of skin between the toes, are all points of +structure which are variable. The period at which the perfect plumage +is acquired varies, as does the state of the down with which the +nestling birds are clothed when hatched. The shape and size of the eggs +vary. The manner of flight differs remarkably; as does in some breeds +the voice and disposition. Lastly, in certain breeds, the males and +females have come to differ to a slight degree from each other. + +Altogether at least a score of pigeons might be chosen, which if shown +to an ornithologist, and he were told that they were wild birds, would +certainly, I think, be ranked by him as well-defined species. Moreover, +I do not believe that any ornithologist would place touch; the +oil-gland is quite aborted. Several other less distinct breeds might +have been specified. +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 might +have called them, could be shown him. + +Great as the differences are between the breeds of pigeons, I am fully +convinced that the common opinion of naturalists is correct, namely, +that all have descended from the rock-pigeon (Columba livia), including +under this term several geographical races or sub-species, which differ +from each other in the most trifling respects. As several of the +reasons which have led me to this belief are in some degree applicable +in other cases, I will here briefly give them. If the several breeds +are not varieties, and have not proceeded from the rock-pigeon, they +must have descended from at least seven or eight aboriginal stocks; for +it is impossible to make the present domestic breeds by the crossing of +any lesser number: how, for instance, could a pouter be produced by +crossing two breeds unless one of the parent-stocks possessed the +characteristic enormous crop? The supposed aboriginal stocks must all +have been rock-pigeons, that is, not breeding or willingly perching on +trees. But besides C. livia, with its geographical sub-species, only +two or three other species of rock-pigeons are known; and these have +not any of the characters of the domestic breeds. Hence the supposed +aboriginal stocks must either still exist in the countries where they +were originally domesticated, and yet be unknown to ornithologists; and +this, considering their size, habits, and remarkable characters, seems +very improbable; or they must have become extinct in the wild state. +But birds breeding on precipices, and good fliers, are unlikely to be +exterminated; and the common rock-pigeon, which has the same habits +with the domestic breeds, has not been exterminated +even on several of the smaller British islets, or on the shores of the +Mediterranean. Hence the supposed extermination of so many species +having similar habits with the rock-pigeon seems to me a very rash +assumption. Moreover, the several above-named domesticated breeds have +been transported to all parts of the world, and, therefore, some of +them must have been carried back again into their native country; but +not one has ever become wild or feral, though the dovecot-pigeon, which +is the rock-pigeon in a very slightly altered state, has become feral +in several places. Again, all recent experience shows that it is most +difficult to get any wild animal to breed freely under domestication; +yet on the hypothesis of the multiple origin of our pigeons, it must be +assumed that at least seven or eight species were so thoroughly +domesticated in ancient times by half-civilized man, as to be quite +prolific under confinement. + +An argument, as it seems to me, of great weight, and applicable in +several other cases, is, that the above-specified breeds, though +agreeing generally in constitution, habits, voice, colouring, and in +most parts of their structure, with the wild rock-pigeon, yet are +certainly highly abnormal in other parts of their structure: we may +look in vain throughout the whole great family of Columbidæ for a beak +like that of the English carrier, or that of the short-faced tumbler, +or barb; for reversed feathers like those of the jacobin; for a crop +like that of the pouter; for tail-feathers like those of the fantail. +Hence it must be assumed not only that half-civilized man succeeded in +thoroughly domesticating several species, but that he intentionally or +by chance picked out extraordinarily abnormal species; and further, +that these very species have since all become extinct or unknown. So +many strange contingencies seem to me improbable in the highest degree. + + +Some facts in regard to the colouring of pigeons well deserve +consideration. The rock-pigeon is of a slaty-blue, and has a white rump +(the Indian sub-species, C. intermedia of Strickland, having it +bluish); the tail has a terminal dark bar, with the bases of the outer +feathers externally edged with white; the wings have two black bars; +some semi-domestic breeds and some apparently truly wild breeds have, +besides the two black bars, the wings chequered with black. These +several marks do not occur together in any other species of the whole +family. Now, in every one of the domestic breeds, taking thoroughly +well-bred birds, all the above marks, even to the white edging of the +outer tail-feathers, sometimes concur perfectly developed. Moreover, +when two birds belonging to two distinct breeds are crossed, neither of +which is blue or has any of the above-specified marks, the mongrel +offspring are very apt suddenly to acquire these characters; for +instance, I crossed some uniformly white fantails with some uniformly +black barbs, and they produced mottled brown and black birds; these I +again crossed together, and one grandchild of the pure white fantail +and pure black barb was of as beautiful a blue colour, with the white +rump, double black wing-bar, and barred and white-edged tail-feathers, +as any wild rock-pigeon! We can understand these facts, on the +well-known principle of reversion to ancestral characters, if all the +domestic breeds have descended from the rock-pigeon. But if we deny +this, we must make one of the two following highly improbable +suppositions. Either, firstly, that all the several imagined aboriginal +stocks were coloured and marked like the rock-pigeon, although no other +existing species is thus coloured and marked, so that in each separate +breed there might be a tendency to revert to the very same colours and +markings. Or, secondly, +that each breed, even the purest, has within a dozen or, at most, +within a score of generations, been crossed by the rock-pigeon: I say +within a dozen or twenty generations, for we know of no fact +countenancing the belief that the child ever reverts to some one +ancestor, removed by a greater number of generations. In a breed which +has been crossed only once with some distinct breed, the tendency to +reversion to any character derived from such cross will naturally +become less and less, as in each succeeding generation there will be +less of the foreign blood; but when there has been no cross with a +distinct breed, and there is a tendency in both parents to revert to a +character, which has been lost during some former generation, this +tendency, for all that we can see to the contrary, may be transmitted +undiminished for an indefinite number of generations. These two +distinct cases are often confounded in treatises on inheritance. + +Lastly, the hybrids or mongrels from between all the domestic breeds of +pigeons are perfectly fertile. I can state this from my own +observations, purposely made on the most distinct breeds. Now, it is +difficult, perhaps impossible, to bring forward one case of the hybrid +offspring of two animals _clearly distinct_ being themselves perfectly +fertile. Some authors believe that long-continued domestication +eliminates this strong tendency to sterility: from the history of the +dog I think there is some probability in this hypothesis, if applied to +species closely related together, though it is unsupported by a single +experiment. But to extend the hypothesis so far as to suppose that +species, aboriginally as distinct as carriers, tumblers, pouters, and +fantails now are, should yield offspring perfectly fertile, _inter se_, +seems to me rash in the extreme. + +From these several reasons, namely, the improbability of man having +formerly got seven or eight supposed +species of pigeons to breed freely under domestication; these supposed +species being quite unknown in a wild state, and their becoming nowhere +feral; these species having very abnormal characters in certain +respects, as compared with all other Columbidæ, though so like in most +other respects to the rock-pigeon; the blue colour and various marks +occasionally appearing in all the breeds, both when kept pure and when +crossed; the mongrel offspring being perfectly fertile;—from these +several reasons, taken together, I can feel no doubt that all our +domestic breeds have descended from the Columba livia with its +geographical sub-species. + +In favour of this view, I may add, firstly, that C. livia, or the +rock-pigeon, has been found capable of domestication in Europe and in +India; and that it agrees in habits and in a great number of points of +structure with all the domestic breeds. Secondly, although an English +carrier or short-faced tumbler differs immensely in certain characters +from the rock-pigeon, yet by comparing the several sub-breeds of these +breeds, more especially those brought from distant countries, we can +make an almost perfect series between the extremes of structure. +Thirdly, those characters which are mainly distinctive of each breed, +for instance the wattle and length of beak of the carrier, the +shortness of that of the tumbler, and the number of tail-feathers in +the fantail, are in each breed eminently variable; and the explanation +of this fact will be obvious when we come to treat of selection. +Fourthly, pigeons have been watched, and tended with the utmost care, +and loved by many people. They have been domesticated for thousands of +years in several quarters of the world; the earliest known record of +pigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as was +pointed out to me by Professor Lepsius; but Mr. Birch informs me that +pigeons are given in a bill +of fare in the previous dynasty. In the time of the Romans, as we hear +from Pliny, immense prices were given for pigeons; “nay, they are come +to this pass, that they can reckon up their pedigree and race.” Pigeons +were much valued by Akber Khan in India, about the year 1600; never +less than 20,000 pigeons were taken with the court. “The monarchs of +Iran and Turan sent him some very rare birds;” and, continues the +courtly historian, “His Majesty by crossing the breeds, which method +was never practised before, has improved them astonishingly.” About +this same period the Dutch were as eager about pigeons as were the old +Romans. The paramount importance of these considerations in explaining +the immense amount of variation which pigeons have undergone, will be +obvious when we treat of Selection. We shall then, also, see how it is +that the breeds so often have a somewhat monstrous character. It is +also a most favourable circumstance for the production of distinct +breeds, that male and female pigeons can be easily mated for life; and +thus different breeds can be kept together in the same aviary. + +I have discussed the probable origin of domestic pigeons at some, yet +quite insufficient, length; because when I first kept pigeons and +watched the several kinds, knowing well how true they bred, I felt +fully as much difficulty in believing that they could ever have +descended from a common parent, as any naturalist could in coming to a +similar conclusion in regard to the many species of finches, or other +large groups of birds, in nature. One circumstance has struck me much; +namely, that all the breeders of the various domestic animals and the +cultivators of plants, with whom I have ever conversed, or whose +treatises I have read, are firmly convinced that the several breeds to +which each has attended, are descended from so many aboriginally +distinct species. +Ask, as I have asked, a celebrated raiser of Hereford cattle, whether +his cattle might not have descended from long horns, and he will laugh +you to scorn. I have never met a pigeon, or poultry, or duck, or rabbit +fancier, who was not fully convinced that each main breed was descended +from a distinct species. Van Mons, in his treatise on pears and apples, +shows how utterly he disbelieves that the several sorts, for instance a +Ribston-pippin or Codlin-apple, could ever have proceeded from the +seeds of the same tree. Innumerable other examples could be given. The +explanation, I think, is simple: from long-continued study they are +strongly impressed with the differences between the several races; and +though they well know that each race varies slightly, for they win +their prizes by selecting such slight differences, yet they ignore all +general arguments, and refuse to sum up in their minds slight +differences accumulated during many successive generations. May not +those naturalists who, knowing far less of the laws of inheritance than +does the breeder, and knowing no more than he does of the intermediate +links in the long lines of descent, yet admit that many of our domestic +races have descended from the same parents—may they not learn a lesson +of caution, when they deride the idea of species in a state of nature +being lineal descendants of other species? + +_Selection_.—Let us now briefly consider the steps by which domestic +races have been produced, either from one or from several allied +species. Some little effect may, perhaps, be attributed to the direct +action of the external conditions of life, and some little to habit; +but he would be a bold man who would account by such agencies for the +differences of a dray and race horse, a greyhound and bloodhound, a +carrier and tumbler pigeon. One of the most remarkable features in our +domesticated races +is that we see in them adaptation, not indeed to the animal’s or +plant’s own good, but to man’s use or fancy. Some variations useful to +him have probably arisen suddenly, or by one step; many botanists, for +instance, believe that the fuller’s teazle, with its hooks, which +cannot be rivalled by any mechanical contrivance, is only a variety of +the wild Dipsacus; and this amount of change may have suddenly arisen +in a seedling. So it has probably been with the turnspit dog; and this +is known to have been the case with the ancon sheep. But when we +compare the dray-horse and race-horse, the dromedary and camel, the +various breeds of sheep fitted either for cultivated land or mountain +pasture, with the wool of one breed good for one purpose, and that of +another breed for another purpose; when we compare the many breeds of +dogs, each good for man in very different ways; when we compare the +game-cock, so pertinacious in battle, with other breeds so little +quarrelsome, with “everlasting layers” which never desire to sit, and +with the bantam so small and elegant; when we compare the host of +agricultural, culinary, orchard, and flower-garden races of plants, +most useful to man at different seasons and for different purposes, or +so beautiful in his eyes, we must, I think, look further than to mere +variability. We cannot suppose that all the breeds were suddenly +produced as perfect and as useful as we now see them; indeed, in +several cases, we know that this has not been their history. The key is +man’s power of accumulative selection: nature gives successive +variations; man adds them up in certain directions useful to him. In +this sense he may be said to make for himself useful breeds. + +The great power of this principle of selection is not hypothetical. It +is certain that several of our eminent breeders have, even within a +single lifetime, modified to +a large extent some breeds of cattle and sheep. In order fully to +realise what they have done, it is almost necessary to read several of +the many treatises devoted to this subject, and to inspect the animals. +Breeders habitually speak of an animal’s organisation as something +quite plastic, which they can model almost as they please. If I had +space I could quote numerous passages to this effect from highly +competent authorities. Youatt, who was probably better acquainted with +the works of agriculturalists than almost any other individual, and who +was himself a very good judge of an animal, speaks of the principle of +selection as “that which enables the agriculturist, not only to modify +the character of his flock, but to change it altogether. It is the +magician’s wand, by means of which he may summon into life whatever +form and mould he pleases.” Lord Somerville, speaking of what breeders +have done for sheep, says:—“It would seem as if they had chalked out +upon a wall a form perfect in itself, and then had given it existence.” +That most skilful breeder, Sir John Sebright, used to say, with respect +to pigeons, that “he would produce any given feather in three years, +but it would take him six years to obtain head and beak.” In Saxony the +importance of the principle of selection in regard to merino sheep is +so fully recognised, that men follow it as a trade: the sheep are +placed on a table and are studied, like a picture by a connoisseur; +this is done three times at intervals of months, and the sheep are each +time marked and classed, so that the very best may ultimately be +selected for breeding. + +What English breeders have actually effected is proved by the enormous +prices given for animals with a good pedigree; and these have now been +exported to almost every quarter of the world. The improvement is by no +means generally due to crossing different breeds; +all the best breeders are strongly opposed to this practice, except +sometimes amongst closely allied sub-breeds. And when a cross has been +made, the closest selection is far more indispensable even than in +ordinary cases. If selection consisted merely in separating some very +distinct variety, and breeding from it, the principle would be so +obvious as hardly to be worth notice; but its importance consists in +the great effect produced by the accumulation in one direction, during +successive generations, of differences absolutely inappreciable by an +uneducated eye—differences which I for one have vainly attempted to +appreciate. Not one man in a thousand has accuracy of eye and judgment +sufficient to become an eminent breeder. If gifted with these +qualities, and he studies his subject for years, and devotes his +lifetime to it with indomitable perseverance, he will succeed, and may +make great improvements; if he wants any of these qualities, he will +assuredly fail. Few would readily believe in the natural capacity and +years of practice requisite to become even a skilful pigeon-fancier. + +The same principles are followed by horticulturists; but the variations +are here often more abrupt. No one supposes that our choicest +productions have been produced by a single variation from the +aboriginal stock. We have proofs that this is not so in some cases, in +which exact records have been kept; thus, to give a very trifling +instance, the steadily-increasing size of the common gooseberry may be +quoted. We see an astonishing improvement in many florists’ flowers, +when the flowers of the present day are compared with drawings made +only twenty or thirty years ago. When a race of plants is once pretty +well established, the seed-raisers do not pick out the best plants, but +merely go over their seed-beds, and pull up the “rogues,” as they call +the plants that deviate from the proper standard. With animals this +kind of selection is, in fact, also followed; for hardly any one is so +careless as to allow his worst animals to breed. + +In regard to plants, there is another means of observing the +accumulated effects of selection—namely, by comparing the diversity of +flowers in the different varieties of the same species in the +flower-garden; the diversity of leaves, pods, or tubers, or whatever +part is valued, in the kitchen-garden, in comparison with the flowers +of the same varieties; and the diversity of fruit of the same species +in the orchard, in comparison with the leaves and flowers of the same +set of varieties. See how different the leaves of the cabbage are, and +how extremely alike the flowers; how unlike the flowers of the +heartsease are, and how alike the leaves; how much the fruit of the +different kinds of gooseberries differ in size, colour, shape, and +hairiness, and yet the flowers present very slight differences. It is +not that the varieties which differ largely in some one point do not +differ at all in other points; this is hardly ever, perhaps never, the +case. The laws of correlation of growth, the importance of which should +never be overlooked, will ensure some differences; but, as a general +rule, I cannot doubt that the continued selection of slight variations, +either in the leaves, the flowers, or the fruit, will produce races +differing from each other chiefly in these characters. + +It may be objected that the principle of selection has been reduced to +methodical practice for scarcely more than three-quarters of a century; +it has certainly been more attended to of late years, and many +treatises have been published on the subject; and the result, I may +add, has been, in a corresponding degree, rapid and important. But it +is very far from true that the principle is a modern discovery. I could +give several references to the full acknowledgment of the importance of +the principle in works of high antiquity. In rude and +barbarous periods of English history choice animals were often +imported, and laws were passed to prevent their exportation: the +destruction of horses under a certain size was ordered, and this may be +compared to the “roguing” of plants by nurserymen. The principle of +selection I find distinctly given in an ancient Chinese encyclopædia. +Explicit rules are laid down by some of the Roman classical writers. +From passages in Genesis, it is clear that the colour of domestic +animals was at that early period attended to. Savages now sometimes +cross their dogs with wild canine animals, to improve the breed, and +they formerly did so, as is attested by passages in Pliny. The savages +in South Africa match their draught cattle by colour, as do some of the +Esquimaux their teams of dogs. Livingstone shows how much good domestic +breeds are valued by the negroes of the interior of Africa who have not +associated with Europeans. Some of these facts do not show actual +selection, but they show that the breeding of domestic animals was +carefully attended to in ancient times, and is now attended to by the +lowest savages. It would, indeed, have been a strange fact, had +attention not been paid to breeding, for the inheritance of good and +bad qualities is so obvious. + +At the present time, eminent breeders try by methodical selection, with +a distinct object in view, to make a new strain or sub-breed, superior +to anything existing in the country. But, for our purpose, a kind of +Selection, which may be called Unconscious, and which results from +every one trying to possess and breed from the best individual animals, +is more important. Thus, a man who intends keeping pointers naturally +tries to get as good dogs as he can, and afterwards breeds from his own +best dogs, but he has no wish or expectation of permanently altering +the breed. Nevertheless I cannot +doubt that this process, continued during centuries, would improve and +modify any breed, in the same way as Bakewell, Collins, etc., by this +very same process, only carried on more methodically, did greatly +modify, even during their own lifetimes, the forms and qualities of +their cattle. Slow and insensible changes of this kind could never be +recognised unless actual measurements or careful drawings of the breeds +in question had been made long ago, which might serve for comparison. +In some cases, however, unchanged or but little changed individuals of +the same breed may be found in less civilised districts, where the +breed has been less improved. There is reason to believe that King +Charles’s spaniel has been unconsciously modified to a large extent +since the time of that monarch. Some highly competent authorities are +convinced that the setter is directly derived from the spaniel, and has +probably been slowly altered from it. It is known that the English +pointer has been greatly changed within the last century, and in this +case the change has, it is believed, been chiefly effected by crosses +with the fox-hound; but what concerns us is, that the change has been +effected unconsciously and gradually, and yet so effectually, that, +though the old Spanish pointer certainly came from Spain, Mr. Borrow +has not seen, as I am informed by him, any native dog in Spain like our +pointer. + +By a similar process of selection, and by careful training, the whole +body of English racehorses have come to surpass in fleetness and size +the parent Arab stock, so that the latter, by the regulations for the +Goodwood Races, are favoured in the weights they carry. Lord Spencer +and others have shown how the cattle of England have increased in +weight and in early maturity, compared with the stock formerly kept in +this country. By comparing the accounts given in old pigeon treatises +of carriers +and tumblers with these breeds as now existing in Britain, India, and +Persia, we can, I think, clearly trace the stages through which they +have insensibly passed, and come to differ so greatly from the +rock-pigeon. + +Youatt gives an excellent illustration of the effects of a course of +selection, which may be considered as unconsciously followed, in so far +that the breeders could never have expected or even have wished to have +produced the result which ensued—namely, the production of two distinct +strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr. +Burgess, as Mr. Youatt remarks, “have been purely bred from the +original stock of Mr. Bakewell for upwards of fifty years. There is not +a suspicion existing in the mind of any one at all acquainted with the +subject that the owner of either of them has deviated in any one +instance from the pure blood of Mr. Bakewell’s flock, and yet the +difference between the sheep possessed by these two gentlemen is so +great that they have the appearance of being quite different +varieties.” + +If there exist savages so barbarous as never to think of the inherited +character of the offspring of their domestic animals, yet any one +animal particularly useful to them, for any special purpose, would be +carefully preserved during famines and other accidents, to which +savages are so liable, and such choice animals would thus generally +leave more offspring than the inferior ones; so that in this case there +would be a kind of unconscious selection going on. We see the value set +on animals even by the barbarians of Tierra del Fuego, by their killing +and devouring their old women, in times of dearth, as of less value +than their dogs. + +In plants the same gradual process of improvement, through the +occasional preservation of the best individuals, whether or not +sufficiently distinct to be ranked +at their first appearance as distinct varieties, and whether or not two +or more species or races have become blended together by crossing, may +plainly be recognised in the increased size and beauty which we now see +in the varieties of the heartsease, rose, pelargonium, dahlia, and +other plants, when compared with the older varieties or with their +parent-stocks. No one would ever expect to get a first-rate heartsease +or dahlia from the seed of a wild plant. No one would expect to raise a +first-rate melting pear from the seed of a wild pear, though he might +succeed from a poor seedling growing wild, if it had come from a +garden-stock. The pear, though cultivated in classical times, appears, +from Pliny’s description, to have been a fruit of very inferior +quality. I have seen great surprise expressed in horticultural works at +the wonderful skill of gardeners, in having produced such splendid +results from such poor materials; but the art, I cannot doubt, has been +simple, and, as far as the final result is concerned, has been followed +almost unconsciously. It has consisted in always cultivating the best +known variety, sowing its seeds, and, when a slightly better variety +has chanced to appear, selecting it, and so onwards. But the gardeners +of the classical period, who cultivated the best pear they could +procure, never thought what splendid fruit we should eat; though we owe +our excellent fruit, in some small degree, to their having naturally +chosen and preserved the best varieties they could anywhere find. + +A large amount of change in our cultivated plants, thus slowly and +unconsciously accumulated, explains, as I believe, the well-known fact, +that in a vast number of cases we cannot recognise, and therefore do +not know, the wild parent-stocks of the plants which have been longest +cultivated in our flower and kitchen gardens. If it has taken centuries +or thousands of years to improve +or modify most of our plants up to their present standard of usefulness +to man, we can understand how it is that neither Australia, the Cape of +Good Hope, nor any other region inhabited by quite uncivilised man, has +afforded us a single plant worth culture. It is not that these +countries, so rich in species, do not by a strange chance possess the +aboriginal stocks of any useful plants, but that the native plants have +not been improved by continued selection up to a standard of perfection +comparable with that given to the plants in countries anciently +civilised. + +In regard to the domestic animals kept by uncivilised man, it should +not be overlooked that they almost always have to struggle for their +own food, at least during certain seasons. And in two countries very +differently circumstanced, individuals of the same species, having +slightly different constitutions or structure, would often succeed +better in the one country than in the other, and thus by a process of +“natural selection,” as will hereafter be more fully explained, two +sub-breeds might be formed. This, perhaps, partly explains what has +been remarked by some authors, namely, that the varieties kept by +savages have more of the character of species than the varieties kept +in civilised countries. + +On the view here given of the all-important part which selection by man +has played, it becomes at once obvious, how it is that our domestic +races show adaptation in their structure or in their habits to man’s +wants or fancies. We can, I think, further understand the frequently +abnormal character of our domestic races, and likewise their +differences being so great in external characters and relatively so +slight in internal parts or organs. Man can hardly select, or only with +much difficulty, any deviation of structure excepting such as is +externally visible; and indeed he rarely cares for what is internal. He +can never act by selection, excepting on variations +which are first given to him in some slight degree by nature. No man +would ever try to make a fantail, till he saw a pigeon with a tail +developed in some slight degree in an unusual manner, or a pouter till +he saw a pigeon with a crop of somewhat unusual size; and the more +abnormal or unusual any character was when it first appeared, the more +likely it would be to catch his attention. But to use such an +expression as trying to make a fantail, is, I have no doubt, in most +cases, utterly incorrect. The man who first selected a pigeon with a +slightly larger tail, never dreamed what the descendants of that pigeon +would become through long-continued, partly unconscious and partly +methodical selection. Perhaps the parent bird of all fantails had only +fourteen tail-feathers somewhat expanded, like the present Java +fantail, or like individuals of other and distinct breeds, in which as +many as seventeen tail-feathers have been counted. Perhaps the first +pouter-pigeon did not inflate its crop much more than the turbit now +does the upper part of its oesophagus,—a habit which is disregarded by +all fanciers, as it is not one of the points of the breed. + +Nor let it be thought that some great deviation of structure would be +necessary to catch the fancier’s eye: he perceives extremely small +differences, and it is in human nature to value any novelty, however +slight, in one’s own possession. Nor must the value which would +formerly be set on any slight differences in the individuals of the +same species, be judged of by the value which would now be set on them, +after several breeds have once fairly been established. Many slight +differences might, and indeed do now, arise amongst pigeons, which are +rejected as faults or deviations from the standard of perfection of +each breed. The common goose has not given rise to any marked +varieties; hence the Thoulouse and the common breed, which differ only +in colour, that +most fleeting of characters, have lately been exhibited as distinct at +our poultry-shows. + +I think these views further explain what has sometimes been +noticed—namely that we know nothing about the origin or history of any +of our domestic breeds. But, in fact, a breed, like a dialect of a +language, can hardly be said to have had a definite origin. A man +preserves and breeds from an individual with some slight deviation of +structure, or takes more care than usual in matching his best animals +and thus improves them, and the improved individuals slowly spread in +the immediate neighbourhood. But as yet they will hardly have a +distinct name, and from being only slightly valued, their history will +be disregarded. When further improved by the same slow and gradual +process, they will spread more widely, and will get recognised as +something distinct and valuable, and will then probably first receive a +provincial name. In semi-civilised countries, with little free +communication, the spreading and knowledge of any new sub-breed will be +a slow process. As soon as the points of value of the new sub-breed are +once fully acknowledged, the principle, as I have called it, of +unconscious selection will always tend,—perhaps more at one period than +at another, as the breed rises or falls in fashion,—perhaps more in one +district than in another, according to the state of 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. + +I must now say a few words on the circumstances, favourable, or the +reverse, to man’s power of selection. A high degree of variability is +obviously favourable, as freely giving the materials for selection to +work on; not that mere individual differences are not amply +sufficient, with extreme care, to allow of the accumulation of a large +amount of modification in almost any desired direction. But as +variations manifestly useful or pleasing to man appear only +occasionally, the chance of their appearance will be much increased by +a large number of individuals being kept; and hence this comes to be of +the highest importance to success. On this principle Marshall has +remarked, with respect to the sheep of parts of Yorkshire, that “as +they generally belong to poor people, and are mostly _in small lots_, +they never can be improved.” On the other hand, nurserymen, from +raising large stocks of the same plants, are generally far more +successful than amateurs in getting new and valuable varieties. The +keeping of a large number of individuals of a species in any country +requires that the species should be placed under favourable conditions +of life, so as to breed freely in that country. When the individuals of +any species are scanty, all the individuals, whatever their quality may +be, will generally be allowed to breed, and this will effectually +prevent selection. But probably the most important point of all, is, +that the animal or plant should be so highly useful to man, or so much +valued by him, that the closest attention should be paid to even the +slightest deviation in the qualities or structure of each individual. +Unless such attention be paid nothing can be effected. I have seen it +gravely remarked, that it was most fortunate that the strawberry began +to vary just when gardeners began to attend closely to this plant. No +doubt the strawberry had always varied since it was cultivated, but the +slight varieties had been neglected. As soon, however, as gardeners +picked out individual plants with slightly larger, earlier, or better +fruit, and raised seedlings from them, and again picked out the best +seedlings and bred from them, then, there appeared (aided by some +crossing with distinct species) those many admirable varieties of the +strawberry which have been raised during the last thirty or forty +years. + +In the case of animals with separate sexes, facility in preventing +crosses is an important element of success in the formation of new +races,—at least, in a country which is already stocked with other +races. In this respect enclosure of the land plays a part. Wandering +savages or the inhabitants of open plains rarely possess more than one +breed of the same species. Pigeons can be mated for life, and this is a +great convenience to the fancier, for thus many races may be kept true, +though mingled in the same aviary; and this circumstance must have +largely favoured the improvement and formation of new breeds. Pigeons, +I may add, can be propagated in great numbers and at a very quick rate, +and inferior birds may be freely rejected, as when killed they serve +for food. On the other hand, cats, from their nocturnal rambling +habits, cannot be matched, and, although so much valued by women and +children, we hardly ever see a distinct breed kept up; such breeds as +we do sometimes see are almost always imported from some other country, +often from islands. Although I do not doubt that some domestic animals +vary less than others, yet the rarity or absence of distinct breeds of +the cat, the donkey, peacock, goose, etc., may be attributed in main +part to selection not having been brought into play: in cats, from the +difficulty in pairing them; in donkeys, from only a few being kept by +poor people, and little attention paid to their breeding; in peacocks, +from not being very easily reared and a large stock not kept; in geese, +from being valuable only for two purposes, food and feathers, and more +especially from no pleasure having been felt in the display of distinct +breeds. + + +To sum up on the origin of our Domestic Races of animals and plants. I +believe that the conditions of life, from their action on the +reproductive system, are so far of the highest importance as causing +variability. I do not believe that variability is an inherent and +necessary contingency, under all circumstances, with all organic +beings, as some authors have thought. The effects of variability are +modified by various degrees of inheritance and of reversion. +Variability is governed by many unknown laws, more especially by that +of correlation of growth. Something may be attributed to the direct +action of the conditions of life. Something must be attributed to use +and disuse. The final result is thus rendered infinitely complex. In +some cases, I do not doubt that the intercrossing of species, +aboriginally distinct, has played an important part in the origin of +our domestic productions. When in any country several domestic breeds +have once been established, their occasional intercrossing, with the +aid of selection, has, no doubt, largely aided in the formation of new +sub-breeds; but the importance of the crossing of varieties has, I +believe, been greatly exaggerated, both in regard to animals and to +those plants which are propagated by seed. In plants which are +temporarily propagated by cuttings, buds, etc., the importance of the +crossing both of distinct species and of varieties is immense; for the +cultivator here quite disregards the extreme variability both of +hybrids and mongrels, and the frequent sterility of hybrids; but the +cases of plants not propagated by seed are of little importance to us, +for their endurance is only temporary. Over all these causes of Change +I am convinced that the accumulative action of Selection, whether +applied methodically and more quickly, or unconsciously and more +slowly, but more efficiently, is by far the predominant Power. + + + + +CHAPTER II. +VARIATION UNDER NATURE. + + +Variability. Individual differences. Doubtful species. Wide ranging, +much diffused, and common species vary most. Species of the larger +genera in any country vary more than the species of the smaller genera. +Many of the species of the larger genera resemble varieties in being +very closely, but unequally, related to each other, and in having +restricted ranges. + +Before applying the principles arrived at in the last chapter to +organic beings in a state of nature, we must briefly discuss whether +these latter are subject to any variation. To treat this subject at all +properly, a long catalogue of dry facts should be given; but these I +shall reserve for my future work. Nor shall I here discuss the various +definitions which have been given of the term species. No one +definition has as yet satisfied all naturalists; yet every naturalist +knows vaguely what he means when he speaks of a species. Generally the +term includes the unknown element of a distinct act of creation. The +term “variety” is almost equally difficult to define; but here +community of descent is almost universally implied, though it can +rarely be proved. We have also what are called monstrosities; but they +graduate into varieties. By a monstrosity I presume is meant some +considerable deviation of structure in one part, either injurious to or +not useful to the species, and not generally propagated. Some authors +use the term “variation” in a technical sense, as implying a +modification directly due to the physical conditions of life; and +“variations” in this sense are supposed not to be inherited: but who +can say that the dwarfed condition of shells in the brackish waters of +the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of +an animal from far northwards, would not in some cases be inherited for +at least some few generations? and in this case I presume that the form +would be called a variety. + +Again, we have many slight differences which may be called individual +differences, such as are known frequently to appear in the offspring +from the same parents, or which may be presumed to have thus arisen, +from being frequently observed in the individuals of the same species +inhabiting the same confined locality. No one supposes that all the +individuals of the same species are cast in the very same mould. These +individual differences are highly important for us, as they afford +materials for natural selection to accumulate, in the same manner as +man can accumulate in any given direction individual differences in his +domesticated productions. These individual differences generally affect +what naturalists consider unimportant parts; but I could show by a long +catalogue of facts, that parts which must be called important, whether +viewed under a physiological or classificatory point of view, sometimes +vary in the individuals of the same species. I am convinced that the +most experienced naturalist would be surprised at the number of the +cases of variability, even in important parts of structure, which he +could collect on good authority, as I have collected, during a course +of years. It should be remembered that systematists are far from +pleased at finding variability in important characters, and that there +are not many men who will laboriously examine internal and important +organs, and compare them in many specimens of the same species. I +should never have expected that the branching of the main nerves close +to the great central ganglion of an insect would have been variable in +the same species; I should have expected that changes of this nature +could have been effected only by slow degrees: yet quite recently Mr. +Lubbock has shown a degree of variability in these main nerves in +Coccus, which may almost be compared to the irregular branching of the +stem of a tree. This philosophical naturalist, I may add, has also +quite recently shown that the muscles in the larvæ of certain insects +are very far from uniform. Authors sometimes argue in a circle when +they state that important organs never vary; for these same authors +practically rank that character as important (as some few naturalists +have honestly confessed) which does not vary; and, under this point of +view, no instance of an important part varying will ever be found: but +under any other point of view many instances assuredly can be given. + +There is one point connected with individual differences, which seems +to me extremely perplexing: I refer to those genera which have +sometimes been called “protean” or “polymorphic,” in which the species +present an inordinate amount of variation; and hardly two naturalists +can agree which forms to rank as species and which as varieties. We may +instance Rubus, Rosa, and Hieracium amongst plants, several genera of +insects, and several genera of Brachiopod shells. In most polymorphic +genera some of the species have fixed and definite characters. Genera +which are polymorphic in one country seem to be, with some few +exceptions, polymorphic in other countries, and likewise, judging from +Brachiopod shells, at former periods of time. These facts seem to be +very perplexing, for they seem to show that this kind of variability is +independent of the conditions of life. I am inclined to suspect that we +see in these polymorphic genera variations in points of structure which +are of no service or disservice to the species, and which consequently +have not been seized on and rendered definite by natural selection, as +hereafter will be explained. + + +Those forms which possess in some considerable degree the character of +species, but which are so closely similar to some other forms, or are +so closely linked to them by intermediate gradations, that naturalists +do not like to rank them as distinct species, are in several respects +the most important for us. We have every reason to believe that many of +these doubtful and closely-allied forms have permanently retained their +characters in their own country for a long time; for as long, as far as +we know, as have good and true species. Practically, when a naturalist +can unite two forms together by others having intermediate characters, +he treats the one as a variety of the other, ranking the most common, +but sometimes the one first described, as the species, and the other as +the variety. But cases of great difficulty, which I will not here +enumerate, sometimes occur in deciding whether or not to rank one form +as a variety of another, even when they are closely connected by +intermediate links; nor will the commonly-assumed hybrid nature of the +intermediate links always remove the difficulty. In very many cases, +however, one form is ranked as a variety of another, not because the +intermediate links have actually been found, but because analogy leads +the observer to suppose either that they do now somewhere exist, or may +formerly have existed; and here a wide door for the entry of doubt and +conjecture is opened. + +Hence, in determining whether a form should be ranked as a species or a +variety, the opinion of naturalists having sound judgment and wide +experience seems the only guide to follow. We must, however, in many +cases, decide by a majority of naturalists, for few well-marked and +well-known varieties can be named which have not been ranked as species +by at least some competent judges. + + +That varieties of this doubtful nature are far from uncommon cannot be +disputed. Compare the several floras of Great Britain, of France or of +the United States, drawn up by different botanists, and see what a +surprising number of forms have been ranked by one botanist as good +species, and by another as mere varieties. Mr. H. C. Watson, to whom I +lie under deep obligation for assistance of all kinds, has marked for +me 182 British plants, which are generally considered as varieties, but +which have all been ranked by botanists as species; and in making this +list he has omitted many trifling varieties, but which nevertheless +have been ranked by some botanists as species, and he has entirely +omitted several highly polymorphic genera. Under genera, including the +most polymorphic forms, Mr. Babington gives 251 species, whereas Mr. +Bentham gives only 112,—a difference of 139 doubtful forms! Amongst +animals which unite for each birth, and which are highly locomotive, +doubtful forms, ranked by one zoologist as a species and by another as +a variety, can rarely be found within the same country, but are common +in separated areas. How many of those birds and insects in North +America and Europe, which differ very slightly from each other, have +been ranked by one eminent naturalist as undoubted species, and by +another as varieties, or, as they are often called, as geographical +races! Many years ago, when comparing, and seeing others compare, the +birds from the separate islands of the Galapagos Archipelago, both one +with another, and with those from the American mainland, I was much +struck how entirely vague and arbitrary is the distinction between +species and varieties. On the islets of the little Madeira group there +are many insects which are characterized as varieties in Mr. +Wollaston’s admirable work, but which it cannot +be doubted would be ranked as distinct species by many entomologists. +Even Ireland has a few animals, now generally regarded as varieties, +but which have been ranked as species by some zoologists. Several most +experienced ornithologists consider our British red grouse as only a +strongly-marked race of a Norwegian species, whereas the greater number +rank it as an undoubted species peculiar to Great Britain. A wide +distance between the homes of two doubtful forms leads many naturalists +to rank both as distinct species; but what distance, it has been well +asked, will suffice? if that between America and Europe is ample, will +that between the Continent and the Azores, or Madeira, or the Canaries, +or Ireland, be sufficient? It must be admitted that many forms, +considered by highly-competent judges as varieties, have so perfectly +the character of species that they are ranked by other highly-competent +judges as good and true species. But to discuss whether they are +rightly called species or varieties, before any definition of these +terms has been generally accepted, is vainly to beat the air. + +Many of the cases of strongly-marked varieties or doubtful species well +deserve consideration; for several interesting lines of argument, from +geographical distribution, analogical variation, hybridism, etc., have +been brought to bear on the attempt to determine their rank. I will +here give only a single instance,—the well-known one of the primrose +and cowslip, or Primula veris and elatior. These plants differ +considerably in appearance; they have a different flavour and emit a +different odour; they flower at slightly different periods; they grow +in somewhat different stations; they ascend mountains to different +heights; they have different geographical ranges; and lastly, according +to very numerous experiments made during several years by +that most careful observer Gärtner, they can be crossed only with much +difficulty. We could hardly wish for better evidence of the two forms +being specifically distinct. On the other hand, they are united by many +intermediate links, and it is very doubtful whether these links are +hybrids; and there is, as it seems to me, an overwhelming amount of +experimental evidence, showing that they descend from common parents, +and consequently must be ranked as varieties. + +Close investigation, in most cases, will bring naturalists to an +agreement how to rank doubtful forms. Yet it must be confessed, that it +is in the best-known countries that we find the greatest number of +forms of doubtful value. I have been struck with the fact, that if any +animal or plant in a state of nature be highly useful to man, or from +any cause closely attract his attention, varieties of it will almost +universally be found recorded. These varieties, moreover, will be often +ranked by some authors as species. Look at the common oak, how closely +it has been studied; yet a German author makes more than a dozen +species out of forms, which are very generally considered as varieties; +and in this country the highest botanical authorities and practical men +can be quoted to show that the sessile and pedunculated oaks are either +good and distinct species or mere varieties. + +When a young naturalist commences the study of a group of organisms +quite unknown to him, he is at first much perplexed to determine what +differences to consider as specific, and what as varieties; for he +knows nothing of the amount and kind of variation to which the group is +subject; and this shows, at least, how very generally there is some +variation. But if he confine his attention to one class within one +country, he will soon make up his mind how to rank most of the doubtful +forms. His +general tendency will be to make many species, for he will become +impressed, just like the pigeon or poultry-fancier before alluded to, +with the amount of difference in the forms which he is continually +studying; and he has little general knowledge of analogical variation +in other groups and in other countries, by which to correct his first +impressions. As he extends the range of his observations, he will meet +with more cases of difficulty; for he will encounter a greater number +of closely-allied forms. But if his observations be widely extended, he +will in the end generally be enabled to make up his own mind which to +call varieties and which species; but he will succeed in this at the +expense of admitting much variation,—and the truth of this admission +will often be disputed by other naturalists. When, moreover, he comes +to study allied forms brought from countries not now continuous, in +which case he can hardly hope to find the intermediate links between +his doubtful forms, he will have to trust almost entirely to analogy, +and his difficulties will rise to a climax. + +Certainly no clear line of demarcation has as yet been drawn between +species and sub-species—that is, the forms which in the opinion of some +naturalists come very near to, but do not quite arrive at the rank of +species; or, again, between sub-species and well-marked varieties, or +between lesser varieties and individual differences. These differences +blend into each other in an insensible series; and a series impresses +the mind with the idea of an actual passage. + +Hence I look at individual differences, though of small interest to the +systematist, as of high importance for us, as being the first step +towards such slight varieties as are barely thought worth recording in +works on natural history. And I look at varieties which are in any +degree more distinct and permanent, as steps leading to more +strongly marked and more permanent varieties; and at these latter, as +leading to sub-species, and to species. The passage from one stage of +difference to another and higher stage may be, in some cases, due +merely to the long-continued action of different physical conditions in +two different regions; but I have not much faith in this view; and I +attribute the passage of a variety, from a state in which it differs +very slightly from its parent to one in which it differs more, to the +action of natural selection in accumulating (as will hereafter be more +fully explained) differences of structure in certain definite +directions. Hence I believe a well-marked variety may be justly called +an incipient species; but whether this belief be justifiable must be +judged of by the general weight of the several facts and views given +throughout this work. + +It need not be supposed that all varieties or incipient species +necessarily attain the rank of species. They may whilst in this +incipient state become extinct, or they may endure as varieties for +very long periods, as has been shown to be the case by Mr. Wollaston +with the varieties of certain fossil land-shells in Madeira. If a +variety were to flourish so as to exceed in numbers the parent species, +it would then rank as the species, and the species as the variety; or +it might come to supplant and exterminate the parent species; or both +might co-exist, and both rank as independent species. But we shall +hereafter have to return to this subject. + +From these remarks it will be seen that I look at the term species, as +one arbitrarily given for the sake of convenience to a set of +individuals closely resembling each other, and that it does not +essentially differ from the term variety, which is given to less +distinct and more fluctuating forms. The term variety, again, in +comparison with mere individual differences, is also applied +arbitrarily, and for mere convenience sake. + + +Guided by theoretical considerations, I thought that some interesting +results might be obtained in regard to the nature and relations of the +species which vary most, by tabulating all the varieties in several +well-worked floras. At first this seemed a simple task; but Mr. H. C. +Watson, to whom I am much indebted for valuable advice and assistance +on this subject, soon convinced me that there were many difficulties, +as did subsequently Dr. Hooker, even in stronger terms. I shall reserve +for my future work the discussion of these difficulties, and the tables +themselves of the proportional numbers of the varying species. Dr. +Hooker permits me to add, that after having carefully read my +manuscript, and examined the tables, he thinks that the following +statements are fairly well established. The whole subject, however, +treated as it necessarily here is with much brevity, is rather +perplexing, and allusions cannot be avoided to the “struggle for +existence,” “divergence of character,” and other questions, hereafter +to be discussed. + +Alph. De Candolle and others have shown that plants which have very +wide ranges generally present varieties; and this might have been +expected, as they become exposed to diverse physical conditions, and as +they come into competition (which, as we shall hereafter see, is a far +more important circumstance) with different sets of organic beings. But +my tables further show that, in any limited country, the species which +are most common, that is abound most in individuals, and the species +which are most widely diffused within their own country (and this is a +different consideration from wide range, and to a certain extent from +commonness), often give rise to varieties sufficiently well-marked to +have been recorded in botanical works. Hence it is the most +flourishing, or, as they may be called, the dominant species,—those +which range widely over the world, are the most diffused in their own +country, and are the most numerous in individuals,—which oftenest +produce well-marked varieties, or, as I consider them, incipient +species. And this, perhaps, might have been anticipated; for, as +varieties, in order to become in any degree permanent, necessarily have +to struggle with the other inhabitants of the country, the species +which are already dominant will be the most likely to yield offspring +which, though in some slight degree modified, will still inherit those +advantages that enabled their parents to become dominant over their +compatriots. + +If the plants inhabiting a country and described in any Flora be +divided into two equal masses, all those in the larger genera being +placed on one side, and all those in the smaller genera on the other +side, a somewhat larger number of the very common and much diffused or +dominant species will be found on the side of the larger genera. This, +again, might have been anticipated; for the mere fact of many species +of the same genus inhabiting any country, shows that there is something +in the organic or inorganic conditions of that country favourable to +the genus; and, consequently, we might have expected to have found in +the larger genera, or those including many species, a large +proportional number of dominant species. But so many causes tend to +obscure this result, that I am surprised that my tables show even a +small majority on the side of the larger genera. I will here allude to +only two causes of obscurity. Fresh-water and salt-loving plants have +generally very wide ranges and are much diffused, but this seems to be +connected with the nature of the stations inhabited by them, and has +little or no relation to the size of the genera to which the species +belong. Again, plants low in the scale of organisation are +generally much more widely diffused than plants higher in the scale; +and here again there is no close relation to the size of the genera. +The cause of lowly-organised plants ranging widely will be discussed in +our chapter on geographical distribution. + +From looking at species as only strongly-marked and well-defined +varieties, I was led to anticipate that the species of the larger +genera in each country would oftener present varieties, than the +species of the smaller genera; for wherever many closely related +species (_i.e._ species of the same genus) have been formed, many +varieties or incipient species ought, as a general rule, to be now +forming. Where many large trees grow, we expect to find saplings. Where +many species of a genus have been formed through variation, +circumstances have been favourable for variation; and hence we might +expect that the circumstances would generally be still favourable to +variation. On the other hand, if we look at each species as a special +act of creation, there is no apparent reason why more varieties should +occur in a group having many species, than in one having few. + +To test the truth of this anticipation I have arranged the plants of +twelve countries, and the coleopterous insects of two districts, into +two nearly equal masses, the species of the larger genera on one side, +and those of the smaller genera on the other side, and it has +invariably proved to be the case that a larger proportion of the +species on the side of the larger genera present varieties, than on the +side of the smaller genera. Moreover, the species of the large genera +which present any varieties, invariably present a larger average number +of varieties than do the species of the small genera. Both these +results follow when another division is made, and when all the smallest +genera, with from only one to four species, are absolutely excluded +from the tables. These +facts are of plain signification on the view that species are only +strongly marked and permanent varieties; for wherever many species of +the same genus have been formed, or where, if we may use the +expression, the manufactory of species has been active, we ought +generally to find the manufactory still in action, more especially as +we have every reason to believe the process of manufacturing new +species to be a slow one. And this certainly is the case, if varieties +be looked at as incipient species; for my tables clearly show as a +general rule that, wherever many species of a genus have been formed, +the species of that genus present a number of varieties, that is of +incipient species, beyond the average. It is not that all large genera +are now varying much, and are thus increasing in the number of their +species, or that no small genera are now varying and increasing; for if +this had been so, it would have been fatal to my theory; inasmuch as +geology plainly tells us that small genera have in the lapse of time +often increased greatly in size; and that large genera have often come +to their maxima, declined, and disappeared. All that we want to show +is, that where many species of a genus have been formed, on an average +many are still forming; and this holds good. + +There are other relations between the species of large genera and their +recorded varieties which deserve notice. We have seen that there is no +infallible criterion by which to distinguish species and well-marked +varieties; and in those cases in which intermediate links have not been +found between doubtful forms, naturalists are compelled to come to a +determination by the amount of difference between them, judging by +analogy whether or not the amount suffices to raise one or both to the +rank of species. Hence the amount of difference is one very important +criterion in settling whether two forms should +be ranked as species or varieties. Now Fries has remarked in regard to +plants, and Westwood in regard to insects, that in large genera the +amount of difference between the species is often exceedingly small. I +have endeavoured to test this numerically by averages, and, as far as +my imperfect results go, they always confirm the view. I have also +consulted some sagacious and most experienced observers, and, after +deliberation, they concur in this view. In this respect, therefore, the +species of the larger genera resemble varieties, more than do the +species of the smaller genera. Or the case may be put in another way, +and it may be said, that in the larger genera, in which a number of +varieties or incipient species greater than the average are now +manufacturing, many of the species already manufactured still to a +certain extent resemble varieties, for they differ from each other by a +less than usual amount of difference. + +Moreover, the species of the large genera are related to each other, in +the same manner as the varieties of any one species are related to each +other. No naturalist pretends that all the species of a genus are +equally distinct from each other; they may generally be divided into +sub-genera, or sections, or lesser groups. As Fries has well remarked, +little groups of species are generally clustered like satellites around +certain other species. And what are varieties but groups of forms, +unequally related to each other, and clustered round certain forms—that +is, round their parent-species? Undoubtedly there is one most important +point of difference between varieties and species; namely, that the +amount of difference between varieties, when compared with each other +or with their parent-species, is much less than that between the +species of the same genus. But when we come to discuss the principle, +as I call it, of Divergence of Character, +we shall see how this may be explained, and how the lesser differences +between varieties will tend to increase into the greater differences +between species. + +There is one other point which seems to me worth notice. Varieties +generally have much restricted ranges: this statement is indeed +scarcely more than a truism, for if a variety were found to have a +wider range than that of its supposed parent-species, their +denominations ought to be reversed. But there is also reason to +believe, that those species which are very closely allied to other +species, and in so far resemble varieties, often have much restricted +ranges. For instance, Mr. H. C. Watson has marked for me in the +well-sifted London Catalogue of plants (4th edition) 63 plants which +are therein ranked as species, but which he considers as so closely +allied to other species as to be of doubtful value: these 63 reputed +species range on an average over 6.9 of the provinces into which Mr. +Watson has divided Great Britain. Now, in this same catalogue, 53 +acknowledged varieties are recorded, and these range over 7.7 +provinces; whereas, the species to which these varieties belong range +over 14.3 provinces. So that the acknowledged varieties have very +nearly the same restricted average range, as have those very closely +allied forms, marked for me by Mr. Watson as doubtful species, but +which are almost universally ranked by British botanists as good and +true species. + +Finally, then, varieties have the same general characters as species, +for they cannot be distinguished from species,—except, firstly, by the +discovery of intermediate linking forms, and the occurrence of such +links cannot affect the actual characters of the forms which they +connect; and except, secondly, by a certain amount of +difference, for two forms, if differing very little, are generally +ranked as varieties, notwithstanding that intermediate linking forms +have not been discovered; but the amount of difference considered +necessary to give to two forms the rank of species is quite indefinite. +In genera having more than the average number of species in any +country, the species of these genera have more than the average number +of varieties. In large genera the species are apt to be closely, but +unequally, allied together, forming little clusters round certain +species. Species very closely allied to other species apparently have +restricted ranges. In all these several respects the species of large +genera present a strong analogy with varieties. And we can clearly +understand these analogies, if species have once existed as varieties, +and have thus originated: whereas, these analogies are utterly +inexplicable if each species has been independently created. + +We have, also, seen that it is the most flourishing and dominant +species of the larger genera which on an average vary most; and +varieties, as we shall hereafter see, tend to become converted into new +and distinct species. The larger genera thus tend to become larger; and +throughout nature the forms of life which are now dominant tend to +become still more dominant by leaving many modified and dominant +descendants. But by steps hereafter to be explained, the larger genera +also tend to break up into smaller genera. And thus, the forms of life +throughout the universe become divided into groups subordinate to +groups. + + + + +CHAPTER III. +STRUGGLE FOR EXISTENCE. + + +Bears on natural selection. The term used in a wide sense. Geometrical +powers of increase. Rapid increase of naturalised animals and plants. +Nature of the checks to increase. Competition universal. Effects of +climate. Protection from the number of individuals. Complex relations +of all animals and plants throughout nature. Struggle for life most +severe between individuals and varieties of the same species; often +severe between species of the same genus. The relation of organism to +organism the most important of all relations. + + +Before entering on the subject of this chapter, I must make a few +preliminary remarks, to show how the struggle for existence bears on +Natural Selection. It has been seen in the last chapter that amongst +organic beings in a state of nature there is some individual +variability; indeed I am not aware that this has ever been disputed. It +is immaterial for us whether a multitude of doubtful forms be called +species or sub-species or varieties; what rank, for instance, the two +or three hundred doubtful forms of British plants are entitled to hold, +if the existence of any well-marked varieties be admitted. But the mere +existence of individual variability and of some few well-marked +varieties, though necessary as the foundation for the work, helps us +but little in understanding how species arise in nature. How have all +those exquisite adaptations of one part of the organisation to another +part, and to the conditions of life, and of one distinct organic being +to another being, been perfected? We see these beautiful co-adaptations +most plainly in the woodpecker and missletoe; and only a little less +plainly in the humblest parasite which clings +to the hairs of a quadruped or feathers of a bird; in the structure of +the beetle which dives through the water; in the plumed seed which is +wafted by the gentlest breeze; in short, we see beautiful adaptations +everywhere and in every part of the organic world. + +Again, it may be asked, how is it that varieties, which I have called +incipient species, become ultimately converted into good and distinct +species, which in most cases obviously differ from each other far more +than do the varieties of the same species? How do those groups of +species, which constitute what are called distinct genera, and which +differ from each other more than do the species of the same genus, +arise? All these results, as we shall more fully see in the next +chapter, follow inevitably from the struggle for life. Owing to this +struggle for life, any variation, however slight and from whatever +cause proceeding, if it be in any degree profitable to an individual of +any species, in its infinitely complex relations to other organic +beings and to external nature, will tend to the preservation of that +individual, and will generally be inherited by its offspring. The +offspring, also, will thus have a better chance of surviving, for, of +the many individuals of any species which are periodically born, but a +small number can survive. I have called this principle, by which each +slight variation, if useful, is preserved, by the term of Natural +Selection, in order to mark its relation to man’s power of selection. +We have seen that man by selection can certainly produce great results, +and can adapt organic beings to his own uses, through the accumulation +of slight but useful variations, given to him by the hand of Nature. +But Natural Selection, as we shall hereafter see, is a power +incessantly ready for action, and is as immeasurably superior to man’s +feeble efforts, as the works of Nature are to those of Art. + + +We will now discuss in a little more detail the struggle for existence. +In my future work this subject shall be treated, as it well deserves, +at much greater length. The elder De Candolle and Lyell have largely +and philosophically shown that all organic beings are exposed to severe +competition. In regard to plants, no one has treated this subject with +more spirit and ability than W. Herbert, Dean of Manchester, evidently +the result of his great horticultural knowledge. Nothing is easier than +to admit in words the truth of the universal struggle for life, or more +difficult—at least I have found it so—than constantly to bear this +conclusion in mind. Yet unless it be thoroughly engrained in the mind, +I am convinced that the whole economy of nature, with every fact on +distribution, rarity, abundance, extinction, and variation, will be +dimly seen or quite misunderstood. We behold the face of nature bright +with gladness, we often see superabundance of food; we do not see, or +we forget, that the birds which are idly singing round us mostly live +on insects or seeds, and are thus constantly destroying life; or we +forget how largely these songsters, or their eggs, or their nestlings, +are destroyed by birds and beasts of prey; we do not always bear in +mind, that though food may be now superabundant, it is not so at all +seasons of each recurring year. + +I should premise that I use the term Struggle for Existence in a large +and metaphorical sense, including dependence of one being on another, +and including (which is more important) not only the life of the +individual, but success in leaving progeny. Two canine animals in a +time of dearth, may be truly said to struggle with each other which +shall get food and live. But a plant on the edge of a desert is said to +struggle for life against the drought, though more properly it should +be said to be dependent on the moisture. A +plant which annually produces a thousand seeds, of which on an average +only one comes to maturity, may be more truly said to struggle with the +plants of the same and other kinds which already clothe the ground. The +missletoe is dependent on the apple and a few other trees, but can only +in a far-fetched sense be said to struggle with these trees, for if too +many of these parasites grow on the same tree, it will languish and +die. But several seedling missletoes, growing close together on the +same branch, may more truly be said to struggle with each other. As the +missletoe is disseminated by birds, its existence depends on birds; and +it may metaphorically be said to struggle with other fruit-bearing +plants, in order to tempt birds to devour and thus disseminate its +seeds rather than those of other plants. In these several senses, which +pass into each other, I use for convenience sake the general term of +struggle for existence. + +A struggle for existence inevitably follows from the high rate at which +all organic beings tend to increase. Every being, which during its +natural lifetime produces several eggs or seeds, must suffer +destruction during some period of its life, and during some season or +occasional year, otherwise, on the principle of geometrical increase, +its numbers would quickly become so inordinately great that no country +could support the product. Hence, as more individuals are produced than +can possibly survive, there must in every case be a struggle for +existence, either one individual with another of the same species, or +with the individuals of distinct species, or with the physical +conditions of life. It is the doctrine of Malthus applied with manifold +force to the whole animal and vegetable kingdoms; for in this case +there can be no artificial increase of food, and no prudential +restraint from marriage. Although some species may be +now increasing, more or less rapidly, in numbers, all cannot do so, for +the world would not hold them. + +There is no exception to the rule that every organic being naturally +increases at so high a rate, that if not destroyed, the earth would +soon be covered by the progeny of a single pair. Even slow-breeding man +has doubled in twenty-five years, and at this rate, in a few thousand +years, there would literally not be standing room for his progeny. +Linnæus has calculated that if an annual plant produced only two +seeds—and there is no plant so unproductive as this—and their seedlings +next year produced two, and so on, then in twenty years there would be +a million plants. The elephant is reckoned to be the slowest breeder of +all known animals, and I have taken some pains to estimate its probable +minimum rate of natural increase: it will be under the mark to assume +that it breeds when thirty years old, and goes on breeding till ninety +years old, bringing forth three pair of young in this interval; if this +be so, at the end of the fifth century there would be alive fifteen +million elephants, descended from the first pair. + +But we have better evidence on this subject than mere theoretical +calculations, namely, the numerous recorded cases of the astonishingly +rapid increase of various animals in a state of nature, when +circumstances have been favourable to them during two or three +following seasons. Still more striking is the evidence from our +domestic animals of many kinds which have run wild in several parts of +the world: if the statements of the rate of increase of slow-breeding +cattle and horses in South America, and latterly in Australia, had not +been well authenticated, they would have been quite 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 +of the plants now most numerous over the wide plains of La Plata, +clothing square leagues of surface almost to the exclusion of all other +plants, have been introduced from Europe; and there are plants which +now range in India, as I hear from Dr. Falconer, from Cape Comorin to +the Himalaya, which have been imported from America since its +discovery. In such cases, and endless instances could be given, no one +supposes that the fertility of these animals or plants has been +suddenly and temporarily increased in any sensible degree. The obvious +explanation is that the conditions of life have been very favourable, +and that there has consequently been less destruction of the old and +young, and that nearly all the young have been enabled to breed. In +such cases the geometrical ratio of increase, the result of which never +fails to be surprising, simply explains the extraordinarily rapid +increase and wide diffusion of naturalised productions in their new +homes. + +In a state of nature almost every plant produces seed, and amongst +animals there are very few which do not annually pair. Hence we may +confidently assert, that all plants and animals are tending to increase +at a geometrical ratio, that all would most rapidly stock every station +in which they could any how exist, and that the geometrical tendency to +increase must be checked by destruction at some period of life. Our +familiarity with the larger domestic animals tends, I think, to mislead +us: we see no great destruction falling on them, and we forget that +thousands are annually slaughtered for food, and that in a state of +nature an equal number would have somehow to be disposed of. + +The only difference between organisms which annually produce eggs or +seeds by the thousand, and those which produce extremely few, is, that +the slow-breeders would require a few more years to people, under +favourable +conditions, a whole district, let it be ever so large. The condor lays +a couple of eggs and the ostrich a score, and yet in the same country +the condor may be the more numerous of the two: the Fulmar petrel lays +but one egg, yet it is believed to be the most numerous bird in the +world. One fly deposits hundreds of eggs, and another, like the +hippobosca, a single one; but this difference does not determine how +many individuals of the two species can be supported in a district. A +large number of eggs is of some importance to those species, which +depend on a rapidly fluctuating amount of food, for it allows them +rapidly to increase in number. But the real importance of a large +number of eggs or seeds is to make up for much destruction at some +period of life; and this period in the great majority of cases is an +early one. If an animal can in any way protect its own eggs or young, a +small number may be produced, and yet the average stock be fully kept +up; but if many eggs or young are destroyed, many must be produced, or +the species will become extinct. It would suffice to keep up the full +number of a tree, which lived on an average for a thousand years, if a +single seed were produced once in a thousand years, supposing that this +seed were never destroyed, and could be ensured to germinate in a +fitting place. So that in all cases, the average number of any animal +or plant depends only indirectly on the number of its eggs or seeds. + +In looking at Nature, it is most necessary to keep the foregoing +considerations always in mind—never to forget that every single organic +being around us may be said to be striving to the utmost to increase in +numbers; that each lives by a struggle at some period of its life; that +heavy destruction inevitably falls either on the young or old, during +each generation or at recurrent intervals. Lighten any check, mitigate +the +destruction ever so little, and the number of the species will almost +instantaneously increase to any amount. The face of Nature may be +compared to a yielding surface, with ten thousand sharp wedges packed +close together and driven inwards by incessant blows, sometimes one +wedge being struck, and then another with greater force. + +What checks the natural tendency of each species to increase in number +is most obscure. Look at the most vigorous species; by as much as it +swarms in numbers, by so much will its tendency to increase be still +further increased. We know not exactly what the checks are in even one +single instance. Nor will this surprise any one who reflects how +ignorant we are on this head, even in regard to mankind, so +incomparably better known than any other animal. This subject has been +ably treated by several authors, and I shall, in my future work, +discuss some of the checks at considerable length, more especially in +regard to the feral animals of South America. Here I will make only a +few remarks, just to recall to the reader’s mind some of the chief +points. Eggs or very young animals seem generally to suffer most, but +this is not invariably the case. With plants there is a vast +destruction of seeds, but, from some observations which I have made, I +believe that it is the seedlings which suffer most from germinating in +ground already thickly stocked with other plants. Seedlings, also, are +destroyed in vast numbers by various enemies; for instance, on a piece +of ground three feet long and two wide, dug and cleared, and where +there could be no choking from other plants, I marked all the seedlings +of our native weeds as they came up, and out of the 357 no less than +295 were destroyed, chiefly by slugs and insects. If turf which has +long been mown, and the case would be the same with turf closely +browsed by quadrupeds, be let to grow, +the more vigorous plants gradually kill the less vigorous, though fully +grown, plants: thus out of twenty species growing on a little plot of +turf (three feet by four) nine species perished from the other species +being allowed to grow up freely. + +The amount of food for each species of course gives the extreme limit +to which each can increase; but very frequently it is not the obtaining +food, but the serving as prey to other animals, which determines the +average numbers of a species. Thus, there seems to be little doubt that +the stock of partridges, grouse, and hares on any large estate depends +chiefly on the destruction of vermin. If not one head of game were shot +during the next twenty years in England, and, at the same time, if no +vermin were destroyed, there would, in all probability, be less game +than at present, although hundreds of thousands of game animals are now +annually killed. On the other hand, in some cases, as with the elephant +and rhinoceros, none are destroyed by beasts of prey: even the tiger in +India most rarely dares to attack a young elephant protected by its +dam. + +Climate plays an important part in determining the average numbers of a +species, and periodical seasons of extreme cold or drought, I believe +to be the most effective of all checks. I estimated that the winter of +1854-55 destroyed four-fifths of the birds in my own grounds; and this +is a tremendous destruction, when we remember that ten per cent. is an +extraordinarily severe mortality from epidemics with man. The action of +climate seems at first sight to be quite independent of the struggle +for existence; but in so far as climate chiefly acts in reducing food, +it brings on the most severe struggle between the individuals, whether +of the same or of distinct species, which subsist on the same kind of +food. Even when climate, for instance extreme +cold, acts directly, it will be the least vigorous, or those which have +got least food through the advancing winter, which will suffer most. +When we travel from south to north, or from a damp region to a dry, we +invariably see some species gradually getting rarer and rarer, and +finally disappearing; and the change of climate being conspicuous, we +are tempted to attribute the whole effect to its direct action. But +this is a very false view: we forget that each species, even where it +most abounds, is constantly suffering enormous destruction at some +period of its life, from enemies or from competitors for the same place +and food; and if these enemies or competitors be in the least degree +favoured by any slight change of climate, they will increase in +numbers, and, as each area is already fully stocked with inhabitants, +the other species will decrease. When we travel southward and see a +species decreasing in numbers, we may feel sure that the cause lies +quite as much in other species being favoured, as in this one being +hurt. So it is when we travel northward, but in a somewhat lesser +degree, for the number of species of all kinds, and therefore of +competitors, decreases northwards; hence in going northward, or in +ascending a mountain, we far oftener meet with stunted forms, due to +the _directly_ injurious action of climate, than we do in proceeding +southwards or in descending a mountain. When we reach the Arctic +regions, or snow-capped summits, or absolute deserts, the struggle for +life is almost exclusively with the elements. + +That climate acts in main part indirectly by favouring other species, +we may clearly see in the prodigious number of plants in our gardens +which can perfectly well endure our climate, but which never become +naturalised, for they cannot compete with our native plants, nor resist +destruction by our native animals. + + +When a species, owing to highly favourable circumstances, increases +inordinately in numbers in a small tract, epidemics—at least, this +seems generally to occur with our game animals—often ensue: and here we +have a limiting check independent of the struggle for life. But even +some of these so-called epidemics appear to be due to parasitic worms, +which have from some cause, possibly in part through facility of +diffusion amongst the crowded animals, been disproportionably favoured: +and here comes in a sort of struggle between the parasite and its prey. + +On the other hand, in many cases, a large stock of individuals of the +same species, relatively to the numbers of its enemies, is absolutely +necessary for its preservation. Thus we can easily raise plenty of corn +and rape-seed, etc., in our fields, because the seeds are in great +excess compared with the number of birds which feed on them; nor can +the birds, though having a superabundance of food at this one season, +increase in number proportionally to the supply of seed, as their +numbers are checked during winter: but any one who has tried, knows how +troublesome it is to get seed from a few wheat or other such plants in +a garden; I have in this case lost every single seed. This view of the +necessity of a large stock of the same species for its preservation, +explains, I believe, some singular facts in nature, such as that of +very rare plants being sometimes extremely abundant in the few spots +where they do occur; and that of some social plants being social, that +is, abounding in individuals, even on the extreme confines of their +range. For in such cases, we may believe, that a plant could exist only +where the conditions of its life were so favourable that many could +exist together, and thus save each other from utter destruction. I +should add that the good effects of frequent intercrossing, and the ill +effects +of close interbreeding, probably come into play in some of these cases; +but on this intricate subject I will not here enlarge. + +Many cases are on record showing how complex and unexpected are the +checks and relations between organic beings, which have to struggle +together in the same country. I will give only a single instance, +which, though a simple one, has interested me. In Staffordshire, on the +estate of a relation where I had ample means of investigation, there +was a large and extremely barren heath, which had never been touched by +the hand of man; but several hundred acres of exactly the same nature +had been enclosed twenty-five years previously and planted with Scotch +fir. The change in the native vegetation of the planted part of the +heath was most remarkable, more than is generally seen in passing from +one quite different soil to another: not only the proportional numbers +of the heath-plants were wholly changed, but twelve species of plants +(not counting grasses and carices) flourished in the plantations, which +could not be found on the heath. The effect on the insects must have +been still greater, for six insectivorous birds were very common in the +plantations, which were not to be seen on the heath; and the heath was +frequented by two or three distinct insectivorous birds. Here we see +how potent has been the effect of the introduction of a single tree, +nothing whatever else having been done, with the exception that the +land had been enclosed, so that cattle could not enter. But how +important an element enclosure is, I plainly saw near Farnham, in +Surrey. Here there are extensive heaths, with a few clumps of old +Scotch firs on the distant hill-tops: within the last ten years large +spaces have been enclosed, and self-sown firs are now springing up in +multitudes, so close together that all cannot live. +When I ascertained that these young trees had not been sown or planted, +I was so much surprised at their numbers that I went to several points +of view, whence I could examine hundreds of acres of the unenclosed +heath, and literally I could not see a single Scotch fir, except the +old planted clumps. But on looking closely between the stems of the +heath, I found a multitude of seedlings and little trees, which had +been perpetually browsed down by the cattle. In one square yard, at a +point some hundred yards distant from one of the old clumps, I counted +thirty-two little trees; and one of them, judging from the rings of +growth, had during twenty-six years tried to raise its head above the +stems of the heath, and had failed. No wonder that, as soon as the land +was enclosed, it became thickly clothed with vigorously growing young +firs. Yet the heath was so extremely barren and so extensive that no +one would ever have imagined that cattle would have so closely and +effectually searched it for food. + +Here we see that cattle absolutely determine the existence of the +Scotch fir; but in several parts of the world insects determine the +existence of cattle. Perhaps Paraguay offers the most curious instance +of this; for here neither cattle nor horses nor dogs have ever run +wild, though they swarm southward and northward in a feral state; and +Azara and Rengger have shown that this is caused by the greater number +in Paraguay of a certain fly, which lays its eggs in the navels of +these animals when first born. The increase of these flies, numerous as +they are, must be habitually checked by some means, probably by birds. +Hence, if certain insectivorous birds (whose numbers are probably +regulated by hawks or beasts of prey) were to increase in Paraguay, the +flies would decrease—then cattle and horses would become feral, and +this would certainly greatly alter (as +indeed I have observed in parts of South America) the vegetation: this +again would largely affect the insects; and this, as we just have seen +in Staffordshire, the insectivorous birds, and so onwards in +ever-increasing circles of complexity. We began this series by +insectivorous birds, and we have ended with them. Not that in nature +the relations can ever be as simple as this. Battle within battle must +ever be recurring with varying success; and yet in the long-run the +forces are so nicely balanced, that the face of nature remains uniform +for long periods of time, though assuredly the merest trifle would +often give the victory to one organic being over another. Nevertheless +so profound is our ignorance, and so high our presumption, that we +marvel when we hear of the extinction of an organic being; and as we do +not see the cause, we invoke cataclysms to desolate the world, or +invent laws on the duration of the forms of life! + +I am tempted to give one more instance showing how plants and animals, +most remote in the scale of nature, are bound together by a web of +complex relations. I shall hereafter have occasion to show that the +exotic Lobelia fulgens, in this part of England, is never visited by +insects, and consequently, from its peculiar structure, never can set a +seed. Many of our orchidaceous plants absolutely require the visits of +moths to remove their pollen-masses and thus to fertilise them. I have, +also, reason to believe that humble-bees are indispensable to the +fertilisation of the heartsease (Viola tricolor), for other bees do not +visit this flower. From experiments which I have tried, I have found +that the visits of bees, if not indispensable, are at least highly +beneficial to the fertilisation of our clovers; but humble-bees alone +visit the common red clover (Trifolium pratense), as other bees cannot +reach the nectar. Hence I have very little doubt, 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 great degree on the number of field-mice, which +destroy their combs and nests; and Mr. H. Newman, who has long attended +to the habits of humble-bees, believes that “more than two thirds of +them are thus destroyed all over England.” Now the number of mice is +largely dependent, as every one knows, on the number of cats; and Mr. +Newman says, “Near villages and small towns I have found the nests of +humble-bees more numerous than elsewhere, which I attribute to the +number of cats that destroy the mice.” Hence it is quite credible that +the presence of a feline animal in large numbers in a district might +determine, through the intervention first of mice and then of bees, the +frequency of certain flowers in that district! + +In the case of every species, many different checks, acting at +different periods of life, and during different seasons or years, +probably come into play; some one check or some few being generally the +most potent, but all concurring in determining the average number or +even the existence of the species. In some cases it can be shown that +widely-different checks act on the same species in different districts. +When we look at the plants and bushes clothing an entangled bank, we +are tempted to attribute their proportional numbers and kinds to what +we call chance. But how false a view is this! Every one has heard that +when an American forest is cut down, a very different vegetation +springs up; but it has been observed that the trees now growing on the +ancient Indian mounds, in the Southern United States, display the same +beautiful diversity and proportion of kinds as in the surrounding +virgin forests. What a struggle between the several kinds of trees +must here have gone on during long centuries, each annually scattering +its seeds by the thousand; what war between insect and insect—between +insects, snails, and other animals with birds and beasts of prey—all +striving to increase, and all feeding on each other or on the trees or +their seeds and seedlings, or on the other plants which first clothed +the ground and thus checked the growth of the trees! Throw up a handful +of feathers, and all must fall to the ground according to definite +laws; but how simple is this problem compared to the action and +reaction of the innumerable plants and animals which have determined, +in the course of centuries, the proportional numbers and kinds of trees +now growing on the old Indian ruins! + +The dependency of one organic being on another, as of a parasite on its +prey, lies generally between beings remote in the scale of nature. This +is often the case with those which may strictly be said to struggle +with each other for existence, as in the case of locusts and +grass-feeding quadrupeds. But the struggle almost invariably will be +most severe between the individuals of the same species, for they +frequent the same districts, require the same food, and are exposed to +the same dangers. In the case of varieties of the same species, the +struggle will generally be almost equally severe, and we sometimes see +the contest soon decided: for instance, if several varieties of wheat +be sown together, and the mixed seed be resown, some of the varieties +which best suit the soil or climate, or are naturally the most fertile, +will beat the others and so yield more seed, and will consequently in a +few years quite supplant the other varieties. To keep up a mixed stock +of even such extremely close varieties as the variously coloured +sweet-peas, they must be each year harvested separately, and the seed +then mixed in due proportion, +otherwise the weaker kinds will steadily decrease in numbers and +disappear. So again with the varieties of sheep: it has been asserted +that certain mountain-varieties will starve out other +mountain-varieties, so that they cannot be kept together. The same +result has followed from keeping together different varieties of the +medicinal leech. It may even be doubted whether the varieties of any +one of our domestic plants or animals have so exactly the same +strength, habits, and constitution, that the original proportions of a +mixed stock could be kept up for half a dozen generations, if they were +allowed to struggle together, like beings in a state of nature, and if +the seed or young were not annually sorted. + +As species of the same genus have usually, though by no means +invariably, some similarity in habits and constitution, and always in +structure, the struggle will generally be more severe between species +of the same genus, when they come into competition with each other, +than between species of distinct genera. We see this in the recent +extension over parts of the United States of one species of swallow +having caused the decrease of another species. The recent increase of +the missel-thrush in parts of Scotland has caused the decrease of the +song-thrush. How frequently we hear of one species of rat taking the +place of another species under the most different climates! In Russia +the small Asiatic cockroach has everywhere driven before it its great +congener. One species of charlock will supplant another, and so in +other cases. We can dimly see why the competition should be most severe +between allied forms, which fill nearly the same place in the economy +of nature; but probably in no one case could we precisely say why one +species has been victorious over another in the great battle of life. + + +A corollary of the highest importance may be deduced from the foregoing +remarks, namely, that the structure of every organic being is related, +in the most essential yet often hidden manner, to that of all other +organic beings, with which it comes into competition for food or +residence, or from which it has to escape, or on which it preys. This +is obvious in the structure of the teeth and talons of the tiger; and +in that of the legs and claws of the parasite which clings to the hair +on the tiger’s body. But in the beautifully plumed seed of the +dandelion, and in the flattened and fringed legs of the water-beetle, +the relation seems at first confined to the elements of air and water. +Yet the advantage of plumed seeds no doubt stands in the closest +relation to the land being already thickly clothed by other plants; so +that the seeds may be widely distributed and fall on unoccupied ground. +In the water-beetle, the structure of its legs, so well adapted for +diving, allows it to compete with other aquatic insects, to hunt for +its own prey, and to escape serving as prey to other animals. + +The store of nutriment laid up within the seeds of many plants seems at +first sight to have no sort of relation to other plants. But from the +strong growth of young plants produced from such seeds (as peas and +beans), when sown in the midst of long grass, I suspect that the chief +use of the nutriment in the seed is to favour the growth of the young +seedling, whilst struggling with other plants growing vigorously all +around. + +Look at a plant in the midst of its range, why does it not double or +quadruple its numbers? We know that it can perfectly well withstand a +little more heat or cold, dampness or dryness, for elsewhere it ranges +into slightly hotter or colder, damper or drier districts. In this case +we can clearly see that if we wished in imagination to give the plant +the power of increasing in number, we should have to give it some +advantage over its competitors, or over the animals which preyed on it. +On the confines of its geographical range, a change of constitution +with respect to climate would clearly be an advantage to our plant; but +we have reason to believe that only a few plants or animals range so +far, that they are destroyed by the rigour of the climate alone. Not +until we reach the extreme confines of life, in the arctic regions or +on the borders of an utter desert, will competition cease. The land may +be extremely cold or dry, yet there will be competition between some +few species, or between the individuals of the same species, for the +warmest or dampest spots. + +Hence, also, we can see that when a plant or animal is placed in a new +country amongst new competitors, though the climate may be exactly the +same as in its former home, yet the conditions of its life will +generally be changed in an essential manner. If we wished to increase +its average numbers in its new home, we should have to modify it in a +different way to what we should have done in its native country; for we +should have to give it some advantage over a different set of +competitors or enemies. + +It is good thus to try in our imagination to give any form some +advantage over another. Probably in no single instance should we know +what to do, so as to succeed. It will convince us of our ignorance on +the mutual relations of all organic beings; a conviction as necessary, +as it seems to be difficult to acquire. All that we can do, is to keep +steadily in mind that each organic being is striving to increase at a +geometrical +ratio; that each at some period of its life, during some season of the +year, during each generation or at intervals, has to struggle for life, +and to suffer great destruction. When we reflect on this struggle, we +may console ourselves with the full belief, that the war of nature is +not incessant, that no fear is felt, that death is generally prompt, +and that the vigorous, the healthy, and the happy survive and multiply. + + + + +CHAPTER IV. +NATURAL SELECTION. + + +Natural Selection: its power compared with man’s selection, its power +on characters of trifling importance, its power at all ages and on both +sexes. Sexual Selection. On the generality of intercrosses between +individuals of the same species. Circumstances favourable and +unfavourable to Natural Selection, namely, intercrossing, isolation, +number of individuals. Slow action. Extinction caused by Natural +Selection. Divergence of Character, related to the diversity of +inhabitants of any small area, and to naturalisation. Action of Natural +Selection, through Divergence of Character and Extinction, on the +descendants from a common parent. Explains the Grouping of all organic +beings. + + +How will the struggle for existence, discussed too briefly in the last +chapter, act in regard to variation? Can the principle of selection, +which we have seen is so potent in the hands of man, apply in nature? I +think we shall see that it can act most effectually. Let it be borne in +mind in what an endless number of strange peculiarities our domestic +productions, and, in a lesser degree, those under nature, vary; and how +strong the hereditary tendency is. Under domestication, it may be truly +said that the whole organisation becomes in some degree plastic. Let it +be borne in mind how infinitely complex and close-fitting are the +mutual relations of all organic beings to each other and to their +physical conditions of life. Can it, then, be thought improbable, +seeing that variations useful to man have undoubtedly occurred, that +other variations useful in some way to each being in the great and +complex battle of life, should sometimes occur in the course of +thousands of generations? If such do occur, can we doubt (remembering +that many more individuals are born than can possibly survive) that +individuals having any advantage, however slight, over others, would +have the best chance of surviving and of procreating their kind? On the +other hand, we may feel sure that any variation in the least degree +injurious would be rigidly destroyed. This preservation of favourable +variations and the rejection of injurious variations, I call Natural +Selection. Variations neither useful nor injurious would not be +affected by natural selection, and would be left a fluctuating element, +as perhaps we see in the species called polymorphic. + +We shall best understand the probable course of natural selection by +taking the case of a country undergoing some physical change, for +instance, of climate. The proportional numbers of its inhabitants would +almost immediately undergo a change, and some species might become +extinct. We may conclude, from what we have seen of the intimate and +complex manner in which the inhabitants of each country are bound +together, that any change in the numerical proportions of some of the +inhabitants, independently of the change of climate itself, would most +seriously affect many of the others. If the country were open on its +borders, new forms would certainly immigrate, and this also would +seriously disturb the relations of some of the former inhabitants. Let +it be remembered how powerful the influence of a single introduced tree +or mammal has been shown to be. But in the case of an island, or of a +country partly surrounded by barriers, into which new and better +adapted forms could not freely enter, we should then have places in the +economy of nature which would assuredly be better filled up, if some of +the original inhabitants were in some manner modified; for, had the +area been open to immigration, these same +places would have been seized on by intruders. In such case, every +slight modification, which in the course of ages chanced to arise, and +which in any way favoured the individuals of any of the species, by +better adapting them to their altered conditions, would tend to be +preserved; and natural selection would thus have free scope for the +work of improvement. + +We have reason to believe, as stated in the first chapter, that a +change in the conditions of life, by specially acting on the +reproductive system, causes or increases variability; and in the +foregoing case the conditions of life are supposed to have undergone a +change, and this would manifestly be favourable to natural selection, +by giving a better chance of profitable variations occurring; and +unless profitable variations do occur, natural selection can do +nothing. Not that, as I believe, any extreme amount of variability is +necessary; as man can certainly produce great results by adding up in +any given direction mere individual differences, so could Nature, but +far more easily, from having incomparably longer time at her disposal. +Nor do I believe that any great physical change, as of climate, or any +unusual degree of isolation to check immigration, is actually necessary +to produce new and unoccupied places for natural selection to fill up +by modifying and improving some of the varying inhabitants. For as all +the inhabitants of each country are struggling together with nicely +balanced forces, extremely slight modifications in the structure or +habits of one inhabitant would often give it an advantage over others; +and still further modifications of the same kind would often still +further increase the advantage. No country can be named in which all +the native inhabitants are now so perfectly adapted to each other and +to the physical conditions under which they live, that none of +them could anyhow be improved; for in all countries, the natives have +been so far conquered by naturalised productions, that they have +allowed foreigners to take firm possession of the land. And as +foreigners have thus everywhere beaten some of the natives, we may +safely conclude that the natives might have been modified with +advantage, so as to have better resisted such intruders. + +As man can produce and certainly has produced a great result by his +methodical and unconscious means of selection, what may not nature +effect? Man can act only on external and visible characters: nature +cares nothing for appearances, except in so far as they may be useful +to any being. She can act on every internal organ, on every shade of +constitutional difference, on the whole machinery of life. Man selects +only for his own good; Nature only for that of the being which she +tends. Every selected character is fully exercised by her; and the +being is placed under well-suited conditions of life. Man keeps the +natives of many climates in the same country; he seldom exercises each +selected character in some peculiar and fitting manner; he feeds a long +and a short beaked pigeon on the same food; he does not exercise a +long-backed or long-legged quadruped in any peculiar manner; he exposes +sheep with long and short wool to the same climate. He does not allow +the most vigorous males to struggle for the females. He does not +rigidly destroy all inferior animals, but protects during each varying +season, as far as lies in his power, all his productions. He often +begins his selection by some half-monstrous form; or at least by some +modification prominent enough to catch his eye, or to be plainly useful +to him. Under nature, the slightest difference of structure or +constitution may well turn the nicely-balanced scale in the +struggle for life, and so be preserved. How fleeting are the wishes and +efforts of man! how short his time! and consequently how poor will his +products be, compared with those accumulated by nature during whole +geological periods. Can we wonder, then, that nature’s productions +should be far “truer” in character than man’s productions; that they +should be infinitely better adapted to the most complex conditions of +life, and should plainly bear the stamp of far higher workmanship? + +It may be said that natural selection is daily and hourly scrutinising, +throughout the world, every variation, even the slightest; rejecting +that which is bad, preserving and adding up all that is good; silently +and insensibly working, whenever and wherever opportunity offers, at +the improvement of each organic being in relation to its organic and +inorganic conditions of life. We see nothing of these slow changes in +progress, until the hand of time has marked the long lapse of ages, and +then so imperfect is our view into long past geological ages, that we +only see that the forms of life are now different from what they +formerly were. + +Although natural selection can act only through and for the good of +each being, yet characters and structures, which we are apt to consider +as of very trifling importance, may thus be acted on. When we see +leaf-eating insects green, and bark-feeders mottled-grey; the alpine +ptarmigan white in winter, the red-grouse the colour of heather, and +the black-grouse that of peaty earth, we must believe that these tints +are of service to these birds and insects in preserving them from +danger. Grouse, if not destroyed at some period of their lives, would +increase in countless numbers; they are known to suffer largely from +birds of prey; and hawks are guided by eyesight to their prey,—so much +so, that on +parts of the Continent persons are warned not to keep white pigeons, as +being the most liable to destruction. Hence I can see no reason to +doubt that natural selection might be most effective in giving the +proper colour to each kind of grouse, and in keeping that colour, when +once acquired, true and constant. Nor ought we to think that the +occasional destruction of an animal of any particular colour would +produce little effect: we should remember how essential it is in a +flock of white sheep to destroy every lamb with the faintest trace of +black. In plants the down on the fruit and the colour of the flesh are +considered by botanists as characters of the most trifling importance: +yet we hear from an excellent horticulturist, Downing, that in the +United States smooth-skinned fruits suffer far more from a beetle, a +curculio, than those with down; that purple plums suffer far more from +a certain disease than yellow plums; whereas another disease attacks +yellow-fleshed peaches far more than those with other coloured flesh. +If, with all the aids of art, these slight differences make a great +difference in cultivating the several varieties, assuredly, in a state +of nature, where the trees would have to struggle with other trees and +with a host of enemies, such differences would effectually settle which +variety, whether a smooth or downy, a yellow or purple fleshed fruit, +should succeed. + +In looking at many small points of difference between species, which, +as far as our ignorance permits us to judge, seem to be quite +unimportant, we must not forget that climate, food, etc., probably +produce some slight and direct effect. It is, however, far more +necessary to bear in mind that there are many unknown laws of +correlation of growth, which, when one part of the organisation is +modified through variation, and the modifications are accumulated by +natural selection for +the good of the being, will cause other modifications, often of the +most unexpected nature. + +As we see that those variations which under domestication appear at any +particular period of life, tend to reappear in the offspring at the +same period;—for instance, in the seeds of the many varieties of our +culinary and agricultural plants; in the caterpillar and cocoon stages +of the varieties of the silkworm; in the eggs of poultry, and in the +colour of the down of their chickens; in the horns of our sheep and +cattle when nearly adult;—so in a state of nature, natural selection +will be enabled to act on and modify organic beings at any age, by the +accumulation of profitable variations at that age, and by their +inheritance at a corresponding age. If it profit a plant to have its +seeds more and more widely disseminated by the wind, I can see no +greater difficulty in this being effected through natural selection, +than in the cotton-planter increasing and improving by selection the +down in the pods on his cotton-trees. Natural selection may modify and +adapt the larva of an insect to a score of contingencies, wholly +different from those which concern the mature insect. These +modifications will no doubt affect, through the laws of correlation, +the structure of the adult; and probably in the case of those insects +which live only for a few hours, and which never feed, a large part of +their structure is merely the correlated result of successive changes +in the structure of their larvæ. So, conversely, modifications in the +adult will probably often affect the structure of the larva; but in all +cases natural selection will ensure that modifications consequent on +other modifications at a different period of life, shall not be in the +least degree injurious: for if they became so, they would cause the +extinction of the species. + +Natural selection will modify the structure of the +young in relation to the parent, and of the parent in relation to the +young. In social animals it will adapt the structure of each individual +for the benefit of the community; if each in consequence profits by the +selected change. What natural selection cannot do, is to modify the +structure of one species, without giving it any advantage, for the good +of another species; and though statements to this effect may be found +in works of natural history, I cannot find one case which will bear +investigation. A structure used only once in an animal’s whole life, if +of high importance to it, might be modified to any extent by natural +selection; for instance, the great jaws possessed by certain insects, +and used exclusively for opening the cocoon—or the hard tip to the beak +of nestling birds, used for breaking the egg. It has been asserted, +that of the best short-beaked tumbler-pigeons more perish in the egg +than are able to get out of it; so that fanciers assist in the act of +hatching. Now, if nature had to make the beak of a full-grown pigeon +very short for the bird’s own advantage, the process of modification +would be very slow, and there would be simultaneously the most rigorous +selection of the young birds within the egg, which had the most +powerful and hardest beaks, for all with weak beaks would inevitably +perish: or, more delicate and more easily broken shells might be +selected, the thickness of the shell being known to vary like every +other structure. + +_Sexual Selection_.—Inasmuch as peculiarities often appear under +domestication in one sex and become hereditarily attached to that sex, +the same fact probably occurs under nature, and if so, natural +selection will be able to modify one sex in its functional relations to +the other sex, or in relation to wholly different habits of life in the +two sexes, as is sometimes the case +with insects. And this leads me to say a few words on what I call +Sexual Selection. This depends, not on a struggle for existence, but on +a struggle between the males for possession of the females; the result +is not death to the unsuccessful competitor, but few or no offspring. +Sexual selection is, therefore, less rigorous than natural selection. +Generally, the most vigorous males, those which are best fitted for +their places in nature, will leave most progeny. But in many cases, +victory will depend not on general vigour, but on having special +weapons, confined to the male sex. A hornless stag or spurless cock +would have a poor chance of leaving offspring. Sexual selection by +always allowing the victor to breed might surely give indomitable +courage, length to the spur, and strength to the wing to strike in the +spurred leg, as well as the brutal cock-fighter, who knows well that he +can improve his breed by careful selection of the best cocks. How low +in the scale of nature this law of battle descends, I know not; male +alligators have been described as fighting, bellowing, and whirling +round, like Indians in a war-dance, for the possession of the females; +male salmons have been seen fighting all day long; male stag-beetles +often bear wounds from the huge mandibles of other males. The war is, +perhaps, severest between the males of polygamous animals, and these +seem oftenest provided with special weapons. The males of carnivorous +animals are already well armed; though to them and to others, special +means of defence may be given through means of sexual selection, as the +mane to the lion, the shoulder-pad to the boar, and the hooked jaw to +the male salmon; for the shield may be as important for victory, as the +sword or spear. + +Amongst birds, the contest is often of a more peaceful character. All +those who have attended to the subject, +believe that there is the severest rivalry between the males of many +species to attract by singing the females. The rock-thrush of Guiana, +birds of Paradise, and some others, congregate; and successive males +display their gorgeous plumage and perform strange antics before the +females, which standing by as spectators, at last choose the most +attractive partner. Those who have closely attended to birds in +confinement well know that they often take individual preferences and +dislikes: thus Sir R. Heron has described how one pied peacock was +eminently attractive to all his hen birds. It may appear childish to +attribute any effect to such apparently weak means: I cannot here enter +on the details necessary to support this view; but if man can in a +short time give elegant carriage and beauty to his bantams, according +to his standard of beauty, I can see no good reason to doubt that +female birds, by selecting, during thousands of generations, the most +melodious or beautiful males, according to their standard of beauty, +might produce a marked effect. I strongly suspect that some well-known +laws with respect to the plumage of male and female birds, in +comparison with the plumage of the young, can be explained on the view +of plumage having been chiefly modified by sexual selection, acting +when the birds have come to the breeding age or during the breeding +season; the modifications thus produced being inherited at +corresponding ages or seasons, either by the males alone, or by the +males and females; but I have not space here to enter on this subject. + +Thus it is, as I believe, that when the males and females of any animal +have the same general habits of life, but differ in structure, colour, +or ornament, such differences have been mainly caused by sexual +selection; that is, individual males have had, in successive +generations, some slight advantage over other +males, in their weapons, means of defence, or charms; and have +transmitted these advantages to their male offspring. Yet, I would not +wish to attribute all such sexual differences to this agency: for we +see peculiarities arising and becoming attached to the male sex in our +domestic animals (as the wattle in male carriers, horn-like +protuberances in the cocks of certain fowls, etc.), which we cannot +believe to be either useful to the males in battle, or attractive to +the females. We see analogous cases under nature, for instance, the +tuft of hair on the breast of the turkey-cock, which can hardly be +either useful or ornamental to this bird;—indeed, had the tuft appeared +under domestication, it would have been called a monstrosity. + +_Illustrations of the action of Natural Selection_.—In order to make it +clear how, as I believe, natural selection acts, I must beg permission +to give one or two imaginary illustrations. Let us take the case of a +wolf, which preys on various animals, securing some by craft, some by +strength, and some by fleetness; and let us suppose that the fleetest +prey, a deer for instance, had from any change in the country increased +in numbers, or that other prey had decreased in numbers, during that +season of the year when the wolf is hardest pressed for food. I can +under such circumstances see no reason to doubt that the swiftest and +slimmest wolves would have the best chance of surviving, and so be +preserved or selected,—provided always that they retained strength to +master their prey at this or at some other period of the year, when +they might be compelled to prey on other animals. I can see no more +reason to doubt this, than that man can improve the fleetness of his +greyhounds by careful and methodical selection, or by that unconscious +selection which results from each man trying +to keep the best dogs without any thought of modifying the breed. + +Even without any change in the proportional numbers of the animals on +which our wolf preyed, a cub might be born with an innate tendency to +pursue certain kinds of prey. Nor can this be thought very improbable; +for we often observe great differences in the natural tendencies of our +domestic animals; one cat, for instance, taking to catch rats, another +mice; one cat, according to Mr. St. John, bringing home winged game, +another hares or rabbits, and another hunting on marshy ground and +almost nightly catching woodcocks or snipes. The tendency to catch rats +rather than mice is known to be inherited. Now, if any slight innate +change of habit or of structure benefited an individual wolf, it would +have the best chance of surviving and of leaving offspring. Some of its +young would probably inherit the same habits or structure, and by the +repetition of this process, a new variety might be formed which would +either supplant or coexist with the parent-form of wolf. Or, again, the +wolves inhabiting a mountainous district, and those frequenting the +lowlands, would naturally be forced to hunt different prey; and from +the continued preservation of the individuals best fitted for the two +sites, two varieties might slowly be formed. These varieties would +cross and blend where they met; but to this subject of intercrossing we +shall soon have to return. I may add, that, according to Mr. Pierce, +there are two varieties of the wolf inhabiting the Catskill Mountains +in the United States, one with a light greyhound-like form, which +pursues deer, and the other more bulky, with shorter legs, which more +frequently attacks the shepherd’s flocks. + +Let us now take a more complex case. Certain plants excrete a sweet +juice, apparently for the sake of eliminating something injurious from +their sap: this is +effected by glands at the base of the stipules in some Leguminosæ, and +at the back of the leaf of the common laurel. This juice, though small +in quantity, is greedily sought by insects. Let us now suppose a little +sweet juice or nectar to be excreted by the inner bases of the petals +of a flower. In this case insects in seeking the nectar would get +dusted with pollen, and would certainly often transport the pollen from +one flower to the stigma of another flower. The flowers of two distinct +individuals of the same species would thus get crossed; and the act of +crossing, we have good reason to believe (as will hereafter be more +fully alluded to), would produce very vigorous seedlings, which +consequently would have the best chance of flourishing and surviving. +Some of these seedlings would probably inherit the nectar-excreting +power. Those individual flowers which had the largest glands or +nectaries, and which excreted most nectar, would be oftenest visited by +insects, and would be oftenest crossed; and so in the long-run would +gain the upper hand. Those flowers, also, which had their stamens and +pistils placed, in relation to the size and habits of the particular +insects which visited them, so as to favour in any degree the +transportal of their pollen from flower to flower, would likewise be +favoured or selected. We might have taken the case of insects visiting +flowers for the sake of collecting pollen instead of nectar; and as +pollen is formed for the sole object of fertilisation, its destruction +appears a simple loss to the plant; yet if a little pollen were +carried, at first occasionally and then habitually, by the +pollen-devouring insects from flower to flower, and a cross thus +effected, although nine-tenths of the pollen were destroyed, it might +still be a great gain to the plant; and those individuals which +produced more and more pollen, and had larger and larger anthers, would +be selected. + + +When our plant, by this process of the continued preservation or +natural selection of more and more attractive flowers, had been +rendered highly attractive to insects, they would, unintentionally on +their part, regularly carry pollen from flower to flower; and that they +can most effectually do this, I could easily show by many striking +instances. I will give only one—not as a very striking case, but as +likewise illustrating one step in the separation of the sexes of +plants, presently to be alluded to. Some holly-trees bear only male +flowers, which have four stamens producing rather a small quantity of +pollen, and a rudimentary pistil; other holly-trees bear only female +flowers; these have a full-sized pistil, and four stamens with +shrivelled anthers, in which not a grain of pollen can be detected. +Having found a female tree exactly sixty yards from a male tree, I put +the stigmas of twenty flowers, taken from different branches, under the +microscope, and on all, without exception, there were pollen-grains, +and on some a profusion of pollen. As the wind had set for several days +from the female to the male tree, the pollen could not thus have been +carried. The weather had been cold and boisterous, and therefore not +favourable to bees, nevertheless every female flower which I examined +had been effectually fertilised by the bees, accidentally dusted with +pollen, having flown from tree to tree in search of nectar. But to +return to our imaginary case: as soon as the plant had been rendered so +highly attractive to insects that pollen was regularly carried from +flower to flower, another process might commence. No naturalist doubts +the advantage of what has been called the “physiological division of +labour;” hence we may believe that it would be advantageous to a plant +to produce stamens alone in one flower or on one whole plant, and +pistils alone in +another flower or on another plant. In plants under culture and placed +under new conditions of life, sometimes the male organs and sometimes +the female organs become more or less impotent; now if we suppose this +to occur in ever so slight a degree under nature, then as pollen is +already carried regularly from flower to flower, and as a more complete +separation of the sexes of our plant would be advantageous on the +principle of the division of labour, individuals with this tendency +more and more increased, would be continually favoured or selected, +until at last a complete separation of the sexes would be effected. + +Let us now turn to the nectar-feeding insects in our imaginary case: we +may suppose the plant of which we have been slowly increasing the +nectar by continued selection, to be a common plant; and that certain +insects depended in main part on its nectar for food. I could give many +facts, showing how anxious bees are to save time; for instance, their +habit of cutting holes and sucking the nectar at the bases of certain +flowers, which they can, with a very little more trouble, enter by the +mouth. Bearing such facts in mind, I can see no reason to doubt that an +accidental deviation in the size and form of the body, or in the +curvature and length of the proboscis, etc., far too slight to be +appreciated by us, might profit a bee or other insect, so that an +individual so characterised would be able to obtain its food more +quickly, and so have a better chance of living and leaving descendants. +Its descendants would probably inherit a tendency to a similar slight +deviation of structure. The tubes of the corollas of the common red and +incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty +glance appear to differ in length; yet the hive-bee can easily suck the +nectar out of the incarnate clover, but not out of the common red +clover, which is visited by humble-bees alone; so that whole fields of +the red clover offer in vain an abundant supply of precious nectar to +the hive-bee. Thus it might be a great advantage to the hive-bee to +have a slightly longer or differently constructed proboscis. On the +other hand, I have found by experiment that the fertility of clover +greatly depends on bees visiting and moving parts of the corolla, so as +to push the pollen on to the stigmatic surface. Hence, again, if +humble-bees were to become rare in any country, it might be a great +advantage to the red clover to have a shorter or more deeply divided +tube to its corolla, so that the hive-bee could visit its flowers. Thus +I can understand how a flower and a bee might slowly become, either +simultaneously or one after the other, modified and adapted in the most +perfect manner to each other, by the continued preservation of +individuals presenting mutual and slightly favourable deviations of +structure. + +I am well aware that this doctrine of natural selection, exemplified in +the above imaginary instances, is open to the same objections which +were at first urged against Sir Charles Lyell’s noble views on “the +modern changes of the earth, as illustrative of geology;” but we now +very seldom hear the action, for instance, of the coast-waves, called a +trifling and insignificant cause, when applied to the excavation of +gigantic valleys or to the formation of the longest lines of inland +cliffs. Natural selection can act only by the preservation and +accumulation of infinitesimally small inherited modifications, each +profitable to the preserved being; and as modern geology has almost +banished such views as the excavation of a great valley by a single +diluvial wave, so will natural selection, if it be a true principle, +banish the belief of the continued creation of new organic +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 unite for each +birth; but in the case of hermaphrodites this is far from obvious. +Nevertheless I am strongly inclined to believe that with all +hermaphrodites two individuals, either occasionally or habitually, +concur for the reproduction of their kind. This view, I may add, was +first suggested by Andrew Knight. We shall presently see its +importance; but I must here treat the subject with extreme brevity, +though I have the materials prepared for an ample discussion. All +vertebrate animals, all insects, and some other large groups of +animals, pair for each birth. Modern research has much diminished the +number of supposed hermaphrodites, and of real hermaphrodites a large +number pair; that is, two individuals regularly unite for reproduction, +which is all that concerns us. But still there are many hermaphrodite +animals which certainly do not habitually pair, and a vast majority of +plants are hermaphrodites. What reason, it may be asked, is there for +supposing in these cases that two individuals ever concur in +reproduction? As it is impossible here to enter on details, I must +trust to some general considerations alone. + +In the first place, I have collected so large a body of facts, showing, +in accordance with the almost universal belief of breeders, that with +animals and plants a cross between different varieties, or between +individuals of the same variety but of another strain, gives vigour and +fertility to the offspring; and on the other hand, that _close_ +interbreeding diminishes vigour and fertility; that +these facts alone incline me to believe that it is a general law of +nature (utterly ignorant though we be of the meaning of the law) that +no organic being self-fertilises itself for an eternity of generations; +but that a cross with another individual is occasionally—perhaps at +very long intervals—indispensable. + +On the belief that this is a law of nature, we can, I think, understand +several large classes of facts, such as the following, which on any +other view are inexplicable. Every hybridizer knows how unfavourable +exposure to wet is to the fertilisation of a flower, yet what a +multitude of flowers have their anthers and stigmas fully exposed to +the weather! but if an occasional cross be indispensable, the fullest +freedom for the entrance of pollen from another individual will explain +this state of exposure, more especially as the plant’s own anthers and +pistil generally stand so close together that self-fertilisation seems +almost inevitable. Many flowers, on the other hand, have their organs +of fructification closely enclosed, as in the great papilionaceous or +pea-family; but in several, perhaps in all, such flowers, there is a +very curious adaptation between the structure of the flower and the +manner in which bees suck the nectar; for, in doing this, they either +push the flower’s own pollen on the stigma, or bring pollen from +another flower. So necessary are the visits of bees to papilionaceous +flowers, that I have found, by experiments published elsewhere, that +their fertility is greatly diminished if these visits be prevented. +Now, it is scarcely possible that bees should fly from flower to +flower, and not carry pollen from one to the other, to the great good, +as I believe, of the plant. Bees will act like a camel-hair pencil, and +it is quite sufficient just to touch the anthers of one flower and then +the stigma of another with the same brush to ensure fertilisation; but +it must not be +supposed that bees would thus produce a multitude of hybrids between +distinct species; for if you bring on the same brush a plant’s own +pollen and pollen from another species, the former will have such a +prepotent effect, that it will invariably and completely destroy, as +has been shown by Gärtner, any influence from the foreign pollen. + +When the stamens of a flower suddenly spring towards the pistil, or +slowly move one after the other towards it, the contrivance seems +adapted solely to ensure self-fertilisation; and no doubt it is useful +for this end: but, the agency of insects is often required to cause the +stamens to spring forward, as Kölreuter has shown to be the case with +the barberry; and curiously in this very genus, which seems to have a +special contrivance for self-fertilisation, it is well known that if +very closely-allied forms or varieties are planted near each other, it +is hardly possible to raise pure seedlings, so largely do they +naturally cross. In many other cases, far from there being any aids for +self-fertilisation, there are special contrivances, as I could show +from the writings of C. C. Sprengel and from my own observations, which +effectually prevent the stigma receiving pollen from its own flower: +for instance, in Lobelia fulgens, there is a really beautiful and +elaborate contrivance by which every one of the infinitely numerous +pollen-granules are swept out of the conjoined anthers of each flower, +before the stigma of that individual flower is ready to receive them; +and as this flower is never visited, at least in my garden, by insects, +it never sets a seed, though by placing pollen from one flower on the +stigma of another, I raised plenty of seedlings; and whilst another +species of Lobelia growing close by, which is visited by bees, seeds +freely. In very many other cases, though there be no special mechanical +contrivance to prevent the stigma of a flower receiving its own pollen, +yet, as +C. C. Sprengel has shown, and as I can confirm, either the anthers +burst before the stigma is ready for fertilisation, or the stigma is +ready before the pollen of that flower is ready, so that these plants +have in fact separated sexes, and must habitually be crossed. How +strange are these facts! How strange that the pollen and stigmatic +surface of the same flower, though placed so close together, as if for +the very purpose of self-fertilisation, should in so many cases be +mutually useless to each other! How simply are these facts explained on +the view of an occasional cross with a distinct individual being +advantageous or indispensable! + +If several varieties of the cabbage, radish, onion, and of some other +plants, be allowed to seed near each other, a large majority, as I have +found, of the seedlings thus raised will turn out mongrels: for +instance, I raised 233 seedling cabbages from some plants of different +varieties growing near each other, and of these only 78 were true to +their kind, and some even of these were not perfectly true. Yet the +pistil of each cabbage-flower is surrounded not only by its own six +stamens, but by those of the many other flowers on the same plant. How, +then, comes it that such a vast number of the seedlings are +mongrelized? I suspect that it must arise from the pollen of a distinct +_variety_ having a prepotent effect over a flower’s own pollen; and +that this is part of the general law of good being derived from the +intercrossing of distinct individuals of the same species. When +distinct _species_ are crossed the case is directly the reverse, for a +plant’s own pollen is always prepotent over foreign pollen; but to this +subject we shall return in a future chapter. + +In the case of a gigantic tree covered with innumerable flowers, it may +be objected that pollen could seldom be carried from tree to tree, and +at most only from flower +to flower on the same tree, and that flowers on the same tree can be +considered as distinct individuals only in a limited sense. I believe +this objection to be valid, but that nature has largely provided +against it by giving to trees a strong tendency to bear flowers with +separated sexes. When the sexes are separated, although the male and +female flowers may be produced on the same tree, we can see that pollen +must be regularly carried from flower to flower; and this will give a +better chance of pollen being occasionally carried from tree to tree. +That trees belonging to all Orders have their sexes more often +separated than other plants, I find to be the case in this country; and +at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. +Asa Gray those of the United States, and the result was as I +anticipated. On the other hand, Dr. Hooker has recently informed me +that he finds that the rule does not hold in Australia; and I have made +these few remarks on the sexes of trees simply to call attention to the +subject. + +Turning for a very brief space to animals: on the land there are some +hermaphrodites, as land-mollusca and earth-worms; but these all pair. +As yet I have not found a single case of a terrestrial animal which +fertilises itself. We can understand this remarkable fact, which offers +so strong a contrast with terrestrial plants, on the view of an +occasional cross being indispensable, by considering the medium in +which terrestrial animals live, and the nature of the fertilising +element; for we know of no means, analogous to the action of insects +and of the wind in the case of plants, by which an occasional cross +could be effected with terrestrial animals without the concurrence of +two individuals. Of aquatic animals, there are many self-fertilising +hermaphrodites; but here currents in the water offer an obvious means +for an occasional cross. And, as in the case of flowers, I have as yet +failed, after consultation with one of the highest authorities, namely, +Professor Huxley, to discover a single case of an hermaphrodite animal +with the organs of reproduction so perfectly enclosed within the body, +that access from without and the occasional influence of a distinct +individual can be shown to be physically impossible. Cirripedes long +appeared to me to present a case of very great difficulty under this +point of view; but I have been enabled, by a fortunate chance, +elsewhere to prove that two individuals, though both are +self-fertilising hermaphrodites, do sometimes cross. + +It must have struck most naturalists as a strange anomaly that, in the +case of both animals and plants, species of the same family and even of +the same genus, though agreeing closely with each other in almost their +whole organisation, yet are not rarely, some of them hermaphrodites, +and some of them unisexual. But if, in fact, all hermaphrodites do +occasionally intercross with other individuals, the difference between +hermaphrodites and unisexual species, as far as function is concerned, +becomes very small. + +From these several considerations and from the many special facts which +I have collected, but which I am not here able to give, I am strongly +inclined to suspect that, both in the vegetable and animal kingdoms, an +occasional intercross with a distinct individual is a law of nature. I +am well aware that there are, on this view, many cases of difficulty, +some of which I am trying to investigate. Finally then, we may conclude +that in many organic beings, a cross between two individuals is an +obvious necessity for each birth; in many others it occurs perhaps only +at long intervals; but in none, as I suspect, can self-fertilisation go +on for perpetuity. + +_Circumstances favourable to Natural Selection_.—This +is an extremely intricate subject. A large amount of inheritable and +diversified variability is favourable, but I believe mere individual +differences suffice for the work. A large number of individuals, by +giving a better chance for the appearance within any given period of +profitable variations, will compensate for a lesser amount of +variability in each individual, and is, I believe, an extremely +important element of success. Though nature grants vast periods of time +for the work of natural selection, she does not grant an indefinite +period; for as all organic beings are striving, it may be said, to +seize on each place in the economy of nature, if any one species does +not become modified and improved in a corresponding degree with its +competitors, it will soon be exterminated. + +In man’s methodical selection, a breeder selects for some definite +object, and free intercrossing will wholly stop his work. But when many +men, without intending to alter the breed, have a nearly common +standard of perfection, and all try to get and breed from the best +animals, much improvement and modification surely but slowly follow +from this unconscious process of selection, notwithstanding a large +amount of crossing with inferior animals. Thus it will be in nature; +for within a confined area, with some place in its polity not so +perfectly occupied as might be, natural selection will always tend to +preserve all the individuals varying in the right direction, though in +different degrees, so as better to fill up the unoccupied place. But if +the area be large, its several districts will almost certainly present +different conditions of life; and then if natural selection be +modifying and improving a species in the several districts, there will +be intercrossing with the other individuals of the same species on the +confines of each. And in this case the effects of intercrossing can +hardly be counterbalanced +by natural selection always tending to modify all the individuals in +each district in exactly the same manner to the conditions of each; for +in a continuous area, the conditions will generally graduate away +insensibly from one district to another. The intercrossing will most +affect those animals which unite for each birth, which wander much, and +which do not breed at a very quick rate. Hence in animals of this +nature, for instance in birds, varieties will generally be confined to +separated countries; and this I believe to be the case. In +hermaphrodite organisms which cross only occasionally, and likewise in +animals which unite for each birth, but which wander little and which +can increase at a very rapid rate, a new and improved variety might be +quickly formed on any one spot, and might there maintain itself in a +body, so that whatever intercrossing took place would be chiefly +between the individuals of the same new variety. A local variety when +once thus formed might subsequently slowly spread to other districts. +On the above principle, nurserymen always prefer getting seed from a +large body of plants of the same variety, as the chance of +intercrossing with other varieties is thus lessened. + +Even in the case of slow-breeding animals, which unite for each birth, +we must not overrate the effects of intercrosses in retarding natural +selection; for I can bring a considerable catalogue of facts, showing +that within the same area, varieties of the same animal can long remain +distinct, from haunting different stations, from breeding at slightly +different seasons, or from varieties of the same kind preferring to +pair together. + +Intercrossing plays a very important part in nature in keeping the +individuals of the same species, or of the same variety, true and +uniform in character. It will obviously thus act far more efficiently +with those animals +which unite for each birth; but I have already attempted to show that +we have reason to believe that occasional intercrosses take place with +all animals and with all plants. Even if these take place only at long +intervals, I am convinced that the young thus produced will gain so +much in vigour and fertility over the offspring from long-continued +self-fertilisation, that they will have a better chance of surviving +and propagating their kind; and thus, in the long run, the influence of +intercrosses, even at rare intervals, will be great. If there exist +organic beings which never intercross, uniformity of character can be +retained amongst them, as long as their conditions of life remain the +same, only through the principle of inheritance, and through natural +selection destroying any which depart from the proper type; but if +their conditions of life change and they undergo modification, +uniformity of character can be given to their modified offspring, +solely by natural selection preserving the same favourable variations. + +Isolation, also, is an important element in the process of natural +selection. In a confined or isolated area, if not very large, the +organic and inorganic conditions of life will generally be in a great +degree uniform; so that natural selection will tend to modify all the +individuals of a varying species throughout the area in the same manner +in relation to the same conditions. Intercrosses, also, with the +individuals of the same species, which otherwise would have inhabited +the surrounding and differently circumstanced districts, will be +prevented. But isolation probably acts more efficiently in checking the +immigration of better adapted organisms, after any physical change, +such as of climate or elevation of the land, etc.; and thus new places +in the natural economy of the country are left open for the old +inhabitants to struggle for, and become adapted to, through +modifications +in their structure and constitution. Lastly, isolation, by checking +immigration and consequently competition, will give time for any new +variety to be slowly improved; and this may sometimes be of importance +in the production of new species. If, however, an isolated area be very +small, either from being surrounded by barriers, or from having very +peculiar physical conditions, the total number of the individuals +supported on it will necessarily be very small; and fewness of +individuals will greatly retard the production of new species through +natural selection, by decreasing the chance of the appearance of +favourable variations. + +If we turn to nature to test the truth of these remarks, and look at +any small isolated area, such as an oceanic island, although the total +number of the species inhabiting it, will be found to be small, as we +shall see in our chapter on geographical distribution; yet of these +species a very large proportion are endemic,—that is, have been +produced there, and nowhere else. Hence an oceanic island at first +sight seems to have been highly favourable for the production of new +species. But we may thus greatly deceive ourselves, for to ascertain +whether a small isolated area, or a large open area like a continent, +has been most favourable for the production of new organic forms, we +ought to make the comparison within equal times; and this we are +incapable of doing. + +Although I do not doubt that isolation is of considerable importance in +the production of new species, on the whole I am inclined to believe +that largeness of area is of more importance, more especially in the +production of species, which will prove capable of enduring for a long +period, and of spreading widely. Throughout a great and open area, not +only will there be a better chance of favourable variations arising +from the large number of individuals of the same species +there supported, but the conditions of life are infinitely complex from +the large number of already existing species; and if some of these many +species become modified and improved, others will have to be improved +in a corresponding degree or they will be exterminated. Each new form, +also, as soon as it has been much improved, will be able to spread over +the open and continuous area, and will thus come into competition with +many others. Hence more new places will be formed, and the competition +to fill them will be more severe, on a large than on a small and +isolated area. Moreover, great areas, though now continuous, owing to +oscillations of level, will often have recently existed in a broken +condition, so that the good effects of isolation will generally, to a +certain extent, have concurred. Finally, I conclude that, although +small isolated areas probably have been in some respects highly +favourable for the production of new species, yet that the course of +modification will generally have been more rapid on large areas; and +what is more important, that the new forms produced on large areas, +which already have been victorious over many competitors, will be those +that will spread most widely, will give rise to most new varieties and +species, and will thus play an important part in the changing history +of the organic world. + +We can, perhaps, on these views, understand some facts which will be +again alluded to in our chapter on geographical distribution; for +instance, that the productions of the smaller continent of Australia +have formerly yielded, and apparently are now yielding, before those of +the larger Europæo-Asiatic area. Thus, also, it is that continental +productions have everywhere become so largely naturalised on islands. +On a small island, the race for life will have been less severe, and +there will have been less modification and less extermination. +Hence, perhaps, it comes that the flora of Madeira, according to Oswald +Heer, resembles the extinct tertiary flora of Europe. All fresh-water +basins, taken together, make a small area compared with that of the sea +or of the land; and, consequently, the competition between fresh-water +productions will have been less severe than elsewhere; new forms will +have been more slowly formed, and old forms more slowly exterminated. +And it is in fresh water that we find seven genera of Ganoid fishes, +remnants of a once preponderant order: and in fresh water we find some +of the most anomalous forms now known in the world, as the +Ornithorhynchus and Lepidosiren, which, like fossils, connect to a +certain extent orders now widely separated in the natural scale. These +anomalous forms may almost be called living fossils; they have endured +to the present day, from having inhabited a confined area, and from +having thus been exposed to less severe competition. + +To sum up the circumstances favourable and unfavourable to natural +selection, as far as the extreme intricacy of the subject permits. I +conclude, looking to the future, that for terrestrial productions a +large continental area, which will probably undergo many oscillations +of level, and which consequently will exist for long periods in a +broken condition, will be the most favourable for the production of +many new forms of life, likely to endure long and to spread widely. For +the area will first have existed as a continent, and the inhabitants, +at this period numerous in individuals and kinds, will have been +subjected to very severe competition. When converted by subsidence into +large separate islands, there will still exist many individuals of the +same species on each island: intercrossing on the confines of the range +of each species will thus be checked: after physical changes of any +kind, immigration will be prevented, +so that new places in the polity of each island will have to be filled +up by modifications of the old inhabitants; and time will be allowed +for the varieties in each to become well modified and perfected. When, +by renewed elevation, the islands shall be re-converted into a +continental area, there will again be severe competition: the most +favoured or improved varieties will be enabled to spread: there will be +much extinction of the less improved forms, and the relative +proportional numbers of the various inhabitants of the renewed +continent will again be changed; and again there will be a fair field +for natural selection to improve still further the inhabitants, and +thus produce new species. + +That natural selection will always act with extreme slowness, I fully +admit. Its action depends on there being places in the polity of +nature, which can be better occupied by some of the inhabitants of the +country undergoing modification of some kind. The existence of such +places will often depend on physical changes, which are generally very +slow, and on the immigration of better adapted forms having been +checked. But the action of natural selection will probably still +oftener depend on some of the inhabitants becoming slowly modified; the +mutual relations of many of the other inhabitants being thus disturbed. +Nothing can be effected, unless favourable variations occur, and +variation itself is apparently always a very slow process. The process +will often be greatly retarded by free intercrossing. Many will exclaim +that these several causes are amply sufficient wholly to stop the +action of natural selection. I do not believe so. On the other hand, I +do believe that natural selection will always act very slowly, often +only at long intervals of time, and generally on only a very few of the +inhabitants of the same region at the same time. I further believe, +that this very slow, intermittent +action of natural selection accords perfectly well with what geology +tells us of the rate and manner at which the inhabitants of this world +have changed. + +Slow though the process of selection may be, if feeble man can do much +by his powers of artificial selection, I can see no limit to the amount +of change, to the beauty and infinite complexity of the coadaptations +between all organic beings, one with another and with their physical +conditions of life, which may be effected in the long course of time by +nature’s power of selection. + +_Extinction_.—This subject will be more fully discussed in our chapter +on Geology; but it must be here alluded to from being intimately +connected with natural selection. Natural selection acts solely through +the preservation of variations in some way advantageous, which +consequently endure. But as from the high geometrical powers of +increase of all organic beings, each area is already fully stocked with +inhabitants, it follows that as each selected and favoured form +increases in number, so will the less favoured forms decrease and +become rare. Rarity, as geology tells us, is the precursor to +extinction. We can, also, see that any form represented by few +individuals will, during fluctuations in the seasons or in the number +of its enemies, run a good chance of utter extinction. But we may go +further than this; for as new forms are continually and slowly being +produced, unless we believe that the number of specific forms goes on +perpetually and almost indefinitely increasing, numbers inevitably must +become extinct. That the number of specific forms has not indefinitely +increased, geology shows us plainly; and indeed we can see reason why +they should not have thus increased, for the number of places in the +polity of nature is not indefinitely great,—not that we +have any means of knowing that any one region has as yet got its +maximum of species. Probably no region is as yet fully stocked, for at +the Cape of Good Hope, where more species of plants are crowded +together than in any other quarter of the world, some foreign plants +have become naturalised, without causing, as far as we know, the +extinction of any natives. + +Furthermore, the species which are most numerous in individuals will +have the best chance of producing within any given period favourable +variations. We have evidence of this, in the facts given in the second +chapter, showing that it is the common species which afford the +greatest number of recorded varieties, or incipient species. Hence, +rare species will be less quickly modified or improved within any given +period, and they will consequently be beaten in the race for life by +the modified descendants of the commoner species. + +From these several considerations I think it inevitably follows, that +as new species in the course of time are formed through natural +selection, others will become rarer and rarer, and finally extinct. The +forms which stand in closest competition with those undergoing +modification and improvement, will naturally suffer most. And we have +seen in the chapter on the Struggle for Existence that it is the most +closely-allied forms,—varieties of the same species, and species of the +same genus or of related genera,—which, from having nearly the same +structure, constitution, and habits, generally come into the severest +competition with each other. Consequently, each new variety or species, +during the progress of its formation, will generally press hardest on +its nearest kindred, and tend to exterminate them. We see the same +process of extermination amongst our domesticated productions, through +the selection of improved forms by man. Many curious +instances could be given showing how quickly new breeds of cattle, +sheep, and other animals, and varieties of flowers, take the place of +older and inferior kinds. In Yorkshire, it is historically known that +the ancient black cattle were displaced by the long-horns, and that +these “were swept away by the short-horns” (I quote the words of an +agricultural writer) “as if by some murderous pestilence.” + +_Divergence of Character_.—The principle, which I have designated by +this term, is of high importance on my theory, and explains, as I +believe, several important facts. In the first place, varieties, even +strongly-marked ones, though having somewhat of the character of +species—as is shown by the hopeless doubts in many cases how to rank +them—yet certainly differ from each other far less than do good and +distinct species. Nevertheless, according to my view, varieties are +species in the process of formation, or are, as I have called them, +incipient species. How, then, does the lesser difference between +varieties become augmented into the greater difference between species? +That this does habitually happen, we must infer from most of the +innumerable species throughout nature presenting well-marked +differences; whereas varieties, the supposed prototypes and parents of +future well-marked species, present slight and ill-defined differences. +Mere chance, as we may call it, might cause one variety to differ in +some character from its parents, and the offspring of this variety +again to differ from its parent in the very same character and in a +greater degree; but this alone would never account for so habitual and +large an amount of difference as that between varieties of the same +species and species of the same genus. + +As has always been my practice, let us seek light on +this head from our domestic productions. We shall here find something +analogous. A fancier is struck by a pigeon having a slightly shorter +beak; another fancier is struck by a pigeon having a rather longer +beak; and on the acknowledged principle that “fanciers do not and will +not admire a medium standard, but like extremes,” they both go on (as +has actually occurred with tumbler-pigeons) choosing and breeding from +birds with longer and longer beaks, or with shorter and shorter beaks. +Again, we may suppose that at an early period one man preferred swifter +horses; another stronger and more bulky horses. The early differences +would be very slight; in the course of time, from the continued +selection of swifter horses by some breeders, and of stronger ones by +others, the differences would become greater, and would be noted as +forming two sub-breeds; finally, after the lapse of centuries, the +sub-breeds would become converted into two well-established and +distinct breeds. As the differences slowly become greater, the inferior +animals with intermediate characters, being neither very swift nor very +strong, will have been neglected, and will have tended to disappear. +Here, then, we see in man’s productions the action of what may be +called the principle of divergence, causing differences, at first +barely appreciable, steadily to increase, and the breeds to diverge in +character both from each other and from their common parent. + +But how, it may be asked, can any analogous principle apply in nature? +I believe it can and does apply most efficiently, from the simple +circumstance that the more diversified the descendants from any one +species become in structure, constitution, and habits, by so much will +they be better enabled to seize on many and widely diversified places +in the polity of nature, and so be enabled to increase in numbers. + + +We can clearly see this in the case of animals with simple habits. Take +the case of a carnivorous quadruped, of which the number that can be +supported in any country has long ago arrived at its full average. If +its natural powers of increase be allowed to act, it can succeed in +increasing (the country not undergoing any change in its conditions) +only by its varying descendants seizing on places at present occupied +by other animals: some of them, for instance, being enabled to feed on +new kinds of prey, either dead or alive; some inhabiting new stations, +climbing trees, frequenting water, and some perhaps becoming less +carnivorous. The more diversified in habits and structure the +descendants of our carnivorous animal became, the more places they +would be enabled to occupy. What applies to one animal will apply +throughout all time to all animals—that is, if they vary—for otherwise +natural selection can do nothing. So it will be with plants. It has +been experimentally proved, that if a plot of ground be sown with one +species of grass, and a similar plot be sown with several distinct +genera of grasses, a greater number of plants and a greater weight of +dry herbage can thus be raised. The same has been found to hold good +when first one variety and then several mixed varieties of wheat have +been sown on equal spaces of ground. Hence, if any one species of grass +were to go on varying, and those varieties were continually selected +which differed from each other in at all the same manner as distinct +species and genera of grasses differ from each other, a greater number +of individual plants of this species of grass, including its modified +descendants, would succeed in living on the same piece of ground. And +we well know that each species and each variety of grass is annually +sowing almost countless seeds; and thus, as it may be said, is striving +its utmost to increase its numbers. Consequently, +I cannot doubt that in the course of many thousands of generations, the +most distinct varieties of any one species of grass would always have +the best chance of succeeding and of increasing in numbers, and thus of +supplanting the less distinct varieties; and varieties, when rendered +very distinct from each other, take the rank of species. + +The truth of the principle, that the greatest amount of life can be +supported by great diversification of structure, is seen under many +natural circumstances. In an extremely small area, especially if freely +open to immigration, and where the contest between individual and +individual must be severe, we always find great diversity in its +inhabitants. For instance, I found that a piece of turf, three feet by +four in size, which had been exposed for many years to exactly the same +conditions, supported twenty species of plants, and these belonged to +eighteen genera and to eight orders, which shows how much these plants +differed from each other. So it is with the plants and insects on small +and uniform islets; and so in small ponds of fresh water. Farmers find +that they can raise most food by a rotation of plants belonging to the +most different orders: nature follows what may be called a simultaneous +rotation. Most of the animals and plants which live close round any +small piece of ground, could live on it (supposing it not to be in any +way peculiar in its nature), and may be said to be striving to the +utmost to live there; but, it is seen, that where they come into the +closest competition with each other, the advantages of diversification +of structure, with the accompanying differences of habit and +constitution, determine that the inhabitants, which thus jostle each +other most closely, shall, as a general rule, belong to what we call +different genera and orders. + +The same principle is seen in the naturalisation of +plants through man’s agency in foreign lands. It might have been +expected that the plants which have succeeded in becoming naturalised +in any land would generally have been closely allied to the indigenes; +for these are commonly looked at as specially created and adapted for +their own country. It might, also, perhaps have been expected that +naturalised plants would have belonged to a few groups more especially +adapted to certain stations in their new homes. But the case is very +different; and Alph. De Candolle has well remarked in his great and +admirable work, that floras gain by naturalisation, proportionally with +the number of the native genera and species, far more in new genera +than in new species. To give a single instance: in the last edition of +Dr. Asa Gray’s ‘Manual of the Flora of the Northern United States,’ 260 +naturalised plants are enumerated, and these belong to 162 genera. We +thus see that these naturalised plants are of a highly diversified +nature. They differ, moreover, to a large extent from the indigenes, +for out of the 162 genera, no less than 100 genera are not there +indigenous, and thus a large proportional addition is made to the +genera of these States. + +By considering the nature of the plants or animals which have struggled +successfully with the indigenes of any country, and have there become +naturalised, we can gain some crude idea in what manner some of the +natives would have had to be modified, in order to have gained an +advantage over the other natives; and we may, I think, at least safely +infer that diversification of structure, amounting to new generic +differences, would have been profitable to them. + +The advantage of diversification in the inhabitants of the same region +is, in fact, the same as that of the physiological division of labour +in the organs of the same individual body—a subject so well elucidated +by +Milne Edwards. No physiologist doubts that a stomach by being adapted +to digest vegetable matter alone, or flesh alone, draws most nutriment +from these substances. So in the general economy of any land, the more +widely and perfectly the animals and plants are diversified for +different habits of life, so will a greater number of individuals be +capable of there supporting themselves. A set of animals, with their +organisation but little diversified, could hardly compete with a set +more perfectly diversified in structure. It may be doubted, for +instance, whether the Australian marsupials, which are divided into +groups differing but little from each other, and feebly representing, +as Mr. Waterhouse and others have remarked, our carnivorous, ruminant, +and rodent mammals, could successfully compete with these +well-pronounced orders. In the Australian mammals, we see the process +of diversification in an early and incomplete stage of development. +After the foregoing discussion, which ought to have been much +amplified, we may, I think, assume that the modified descendants of any +one species will succeed by so much the better as they become more +diversified in structure, and are thus enabled to encroach on places +occupied by other beings. Now let us see how this principle of great +benefit being derived from divergence of character, combined with the +principles of natural selection and of extinction, will tend to act. + +The accompanying diagram will aid us in understanding this rather +perplexing subject. Let A to L represent the species of a genus large +in its own country; these species are supposed to resemble each other +in unequal degrees, as is so generally the case in nature, and as is +represented in the diagram by the letters standing at unequal +distances. I have said a large genus, because we have seen in the +second chapter, +that on an average more of the species of large genera vary than of +small genera; and the varying species of the large genera present a +greater number of varieties. We have, also, seen that the species, +which are the commonest and the most widely-diffused, vary more than +rare species with restricted ranges. Let (A) be a common, +widely-diffused, and varying species, belonging to a genus large in its +own country. The little fan of diverging dotted lines of unequal +lengths proceeding from (A), may represent its varying offspring. The +variations are supposed to be extremely slight, but of the most +diversified nature; they are not supposed all to appear simultaneously, +but often after long intervals of time; nor are they all supposed to +endure for equal periods. Only those variations which are in some way +profitable will be preserved or naturally selected. And here the +importance of the principle of benefit being derived from divergence of +character comes in; for this will generally lead to the most different +or divergent variations (represented by the outer dotted lines) being +preserved and accumulated by natural selection. When a dotted line +reaches one of the horizontal lines, and is there marked by a small +numbered letter, a sufficient amount of variation is supposed to have +been accumulated to have formed a fairly well-marked variety, such as +would be thought worthy of record in a systematic work. + +The intervals between the horizontal lines in the diagram, may +represent each a thousand generations; but it would have been better if +each had represented ten thousand generations. After a thousand +generations, species (A) is supposed to have produced two fairly +well-marked varieties, namely _a_1 and _m_1. These two varieties will +generally continue to be exposed to the same conditions which made +their parents variable, +and the tendency to variability is in itself hereditary, consequently +they will tend to vary, and generally to vary in nearly the same manner +as their parents varied. Moreover, these two varieties, being only +slightly modified forms, will tend to inherit those advantages which +made their common parent (A) more numerous than most of the other +inhabitants of the same country; they will likewise partake of those +more general advantages which made the genus to which the +parent-species belonged, a large genus in its own country. And these +circumstances we know to be favourable to the production of new +varieties. + +If, then, these two varieties be variable, the most divergent of their +variations will generally be preserved during the next thousand +generations. And after this interval, variety _a_1 is supposed in the +diagram to have produced variety _a_2, which will, owing to the +principle of divergence, differ more from (A) than did variety _a_1. +Variety _m_1 is supposed to have produced two varieties, namely _m_2 +and _s_2, differing from each other, and more considerably from their +common parent (A). We may continue the process by similar steps for any +length of time; some of the varieties, after each thousand generations, +producing only a single variety, but in a more and more modified +condition, some producing two or three varieties, and some failing to +produce any. Thus the varieties or modified descendants, proceeding +from the common parent (A), will generally go on increasing in number +and diverging in character. In the diagram the process is represented +up to the ten-thousandth generation, and under a condensed and +simplified form up to the fourteen-thousandth generation. + +But I must here remark that I do not suppose that the process ever goes +on so regularly as is represented in the diagram, though in itself made +somewhat irregular. +I am far from thinking that the most divergent varieties will +invariably prevail and multiply: a medium form may often long endure, +and may or may not produce more than one modified descendant; for +natural selection will always act according to the nature of the places +which are either unoccupied or not perfectly occupied by other beings; +and this will depend on infinitely complex relations. But as a general +rule, the more diversified in structure the descendants from any one +species can be rendered, the more places they will be enabled to seize +on, and the more their modified progeny will be increased. In our +diagram the line of succession is broken at regular intervals by small +numbered letters marking the successive forms which have become +sufficiently distinct to be recorded as varieties. But these breaks are +imaginary, and might have been inserted anywhere, after intervals long +enough to have allowed the accumulation of a considerable amount of +divergent variation. + +As all the modified descendants from a common and widely-diffused +species, belonging to a large genus, will tend to partake of the same +advantages which made their parent successful in life, they will +generally go on multiplying in number as well as diverging in +character: this is represented in the diagram by the several divergent +branches proceeding from (A). The modified offspring from the later and +more highly improved branches in the lines of descent, will, it is +probable, often take the place of, and so destroy, the earlier and less +improved branches: this is represented in the diagram by some of the +lower branches not reaching to the upper horizontal lines. In some +cases I do not doubt that the process of modification will be confined +to a single line of descent, and the number of the descendants will not +be increased; although the amount +of divergent modification may have been increased in the successive +generations. This case would be represented in the diagram, if all the +lines proceeding from (A) were removed, excepting that from _a_1 to +_a_10. In the same way, for instance, the English race-horse and +English pointer have apparently both gone on slowly diverging in +character from their original stocks, without either having given off +any fresh branches or races. + +After ten thousand generations, species (A) is supposed to have +produced three forms, _a_10, _f_10, and _m_10, which, from having +diverged in character during the successive generations, will have come +to differ largely, but perhaps unequally, from each other and from +their common parent. If we suppose the amount of change between each +horizontal line in our diagram to be excessively small, these three +forms may still be only well-marked varieties; or they may have arrived +at the doubtful category of sub-species; but we have only to suppose +the steps in the process of modification to be more numerous or greater +in amount, to convert these three forms into well-defined species: thus +the diagram illustrates the steps by which the small differences +distinguishing varieties are increased into the larger differences +distinguishing species. By continuing the same process for a greater +number of generations (as shown in the diagram in a condensed and +simplified manner), we get eight species, marked by the letters between +_a_14 and _m_14, all descended from (A). Thus, as I believe, species +are multiplied and genera are formed. + +In a large genus it is probable that more than one species would vary. +In the diagram I have assumed that a second species (I) has produced, +by analogous steps, after ten thousand generations, either two +well-marked varieties (_w_10 and _z_10) or two species, according to +the amount of change supposed to be represented between +the horizontal lines. After fourteen thousand generations, six new +species, marked by the letters _n_14 to _z_14, are supposed to have +been produced. In each genus, the species, which are already extremely +different in character, will generally tend to produce the greatest +number of modified descendants; for these will have the best chance of +filling new and widely different places in the polity of nature: hence +in the diagram I have chosen the extreme species (A), and the nearly +extreme species (I), as those which have largely varied, and have given +rise to new varieties and species. The other nine species (marked by +capital letters) of our original genus, may for a long period continue +transmitting unaltered descendants; and this is shown in the diagram by +the dotted lines not prolonged far upwards from want of space. + +But during the process of modification, represented in the diagram, +another of our principles, namely that of extinction, will have played +an important part. As in each fully stocked country natural selection +necessarily acts by the selected form having some advantage in the +struggle for life over other forms, there will be a constant tendency +in the improved descendants of any one species to supplant and +exterminate in each stage of descent their predecessors and their +original parent. For it should be remembered that the competition will +generally be most severe between those forms which are most nearly +related to each other in habits, constitution, and structure. Hence all +the intermediate forms between the earlier and later states, that is +between the less and more improved state of a species, as well as the +original parent-species itself, will generally tend to become extinct. +So it probably will be with many whole collateral lines of descent, +which will be conquered by later and improved lines of descent. If, +however, the +modified offspring of a species get into some distinct country, or +become quickly adapted to some quite new station, in which child and +parent do not come into competition, both may continue to exist. + +If then our diagram be assumed to represent a considerable amount of +modification, species (A) and all the earlier varieties will have +become extinct, having been replaced by eight new species (_a_14 to +_m_14); and (I) will have been replaced by six (_n_14 to _z_14) new +species. + +But we may go further than this. The original species of our genus were +supposed to resemble each other in unequal degrees, as is so generally +the case in nature; species (A) being more nearly related to B, C, and +D, than to the other species; and species (I) more to G, H, K, L, than +to the others. These two species (A) and (I), were also supposed to be +very common and widely diffused species, so that they must originally +have had some advantage over most of the other species of the genus. +Their modified descendants, fourteen in number at the +fourteen-thousandth generation, will probably have inherited some of +the same advantages: they have also been modified and improved in a +diversified manner at each stage of descent, so as to have become +adapted to many related places in the natural economy of their country. +It seems, therefore, to me extremely probable that they will have taken +the places of, and thus exterminated, not only their parents (A) and +(I), but likewise some of the original species which were most nearly +related to their parents. Hence very few of the original species will +have transmitted offspring to the fourteen-thousandth generation. We +may suppose that only one (F), of the two species which were least +closely related to the other nine original species, has transmitted +descendants to this late stage of descent. + + +The new species in our diagram descended from the original eleven +species, will now be fifteen in number. Owing to the divergent tendency +of natural selection, the extreme amount of difference in character +between species _a_14 and _z_14 will be much greater than that between +the most different of the original eleven species. The new species, +moreover, will be allied to each other in a widely different manner. Of +the eight descendants from (A) the three marked _a_14, _q_14, _p_14, +will be nearly related from having recently branched off from _a_10; +_b_14 and _f_14, from having diverged at an earlier period from _a_5, +will be in some degree distinct from the three first-named species; and +lastly, _o_14, _e_14, and _m_14, will be nearly related one to the +other, but from having diverged at the first commencement of the +process of modification, will be widely different from the other five +species, and may constitute a sub-genus or even a distinct genus. + +The six descendants from (I) will form two sub-genera or even genera. +But as the original species (I) differed largely from (A), standing +nearly at the extreme points of the original genus, the six descendants +from (I) will, owing to inheritance, differ considerably from the eight +descendants from (A); the two groups, moreover, are supposed to have +gone on diverging in different directions. The intermediate species, +also (and this is a very important consideration), which connected the +original species (A) and (I), have all become, excepting (F), extinct, +and have left no descendants. Hence the six new species descended from +(I), and the eight descended from (A), will have to be ranked as very +distinct genera, or even as distinct sub-families. + +Thus it is, as I believe, that two or more genera are produced by +descent, with modification, from two or more species of the same genus. +And the two or more +parent-species are supposed to have descended from some one species of +an earlier genus. In our diagram, this is indicated by the broken +lines, beneath the capital letters, converging in sub-branches +downwards towards a single point; this point representing a single +species, the supposed single parent of our several new sub-genera and +genera. + +It is worth while to reflect for a moment on the character of the new +species F14, which is supposed not to have diverged much in character, +but to have retained the form of (F), either unaltered or altered only +in a slight degree. In this case, its affinities to the other fourteen +new species will be of a curious and circuitous nature. Having +descended from a form which stood between the two parent-species (A) +and (I), now supposed to be extinct and unknown, it will be in some +degree intermediate in character between the two groups descended from +these species. But as these two groups have gone on diverging in +character from the type of their parents, the new species (F14) will +not be directly intermediate between them, but rather between types of +the two groups; and every naturalist will be able to bring some such +case before his mind. + +In the diagram, each horizontal line has hitherto been supposed to +represent a thousand generations, but each may represent a million or +hundred million generations, and likewise a section of the successive +strata of the earth’s crust including extinct remains. We shall, when +we come to our chapter on Geology, have to refer again to this subject, +and I think we shall then see that the diagram throws light on the +affinities of extinct beings, which, though generally belonging to the +same orders, or families, or genera, with those now living, yet are +often, in some degree, intermediate in character between existing +groups; and we can understand this fact, for +the extinct species lived at very ancient epochs when the branching +lines of descent had diverged less. + +I see no reason to limit the process of modification, as now explained, +to the formation of genera alone. If, in our diagram, we suppose the +amount of change represented by each successive group of diverging +dotted lines to be very great, the forms marked _a_14 to _p_14, those +marked _b_14 and _f_14, and those marked _o_14 to _m_14, will form +three very distinct genera. We shall also have two very distinct genera +descended from (I) and as these latter two genera, both from continued +divergence of character and from inheritance from a different parent, +will differ widely from the three genera descended from (A), the two +little groups of genera will form two distinct families, or even +orders, according to the amount of divergent modification supposed to +be represented in the diagram. And the two new families, or orders, +will have descended from two species of the original genus; and these +two species are supposed to have descended from one species of a still +more ancient and unknown genus. + +We have seen that in each country it is the species of the larger +genera which oftenest present varieties or incipient species. This, +indeed, might have been expected; for as natural selection acts through +one form having some advantage over other forms in the struggle for +existence, it will chiefly act on those which already have some +advantage; and the largeness of any group shows that its species have +inherited from a common ancestor some advantage in common. Hence, the +struggle for the production of new and modified descendants, will +mainly lie between the larger groups, which are all trying to increase +in number. One large group will slowly conquer another large group, +reduce its numbers, and thus lessen its chance of further variation and +improvement. Within the same large +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 tend to +disappear. Looking to the future, we can predict that the groups of +organic beings which are now large and triumphant, and which are least +broken up, that is, which as yet have suffered least extinction, will +for a long period continue to increase. But which groups will +ultimately prevail, no man can predict; for we well know that many +groups, formerly most extensively developed, have now become extinct. +Looking still more remotely to the future, we may predict that, owing +to the continued and steady increase of the larger groups, a multitude +of smaller groups will become utterly extinct, and leave no modified +descendants; and consequently that of the species living at any one +period, extremely few will transmit descendants to a remote futurity. I +shall have to return to this subject in the chapter on Classification, +but I may add that on this view of extremely few of the more ancient +species having transmitted descendants, and on the view of all the +descendants of the same species making a class, we can understand how +it is that there exist but very few classes in each main division of +the animal and vegetable kingdoms. Although extremely few of the most +ancient species may now have living and modified descendants, yet at +the most remote geological period, the earth may have been as well +peopled with many species of many genera, families, orders, and +classes, as at the present day. + +_Summary of the Chapter_.—If during the long course of ages and under +varying conditions of life, organic beings +vary at all in the several parts of their organisation, and I think +this cannot be disputed; if there be, owing to the high geometrical +powers of increase of each species, at some age, season, or year, a +severe struggle for life, and this certainly cannot be disputed; then, +considering the infinite complexity of the relations of all organic +beings to each other and to their conditions of existence, causing an +infinite diversity in structure, constitution, and habits, to be +advantageous to them, I think it would be a most extraordinary fact if +no variation ever had occurred useful to each being’s own welfare, in +the same way as so many variations have occurred useful to man. But if +variations useful to any organic being do occur, assuredly individuals +thus characterised will have the best chance of being preserved in the +struggle for life; and from the strong principle of inheritance they +will tend to produce offspring similarly characterised. This principle +of preservation, I have called, for the sake of brevity, Natural +Selection. Natural selection, on the principle of qualities being +inherited at corresponding ages, can modify the egg, seed, or young, as +easily as the adult. Amongst many animals, sexual selection will give +its aid to ordinary selection, by assuring to the most vigorous and +best adapted males the greatest number of offspring. Sexual selection +will also give characters useful to the males alone, in their struggles +with other males. + +Whether natural selection has really thus acted in nature, in modifying +and adapting the various forms of life to their several conditions and +stations, must be judged of by the general tenour and balance of +evidence given in the following chapters. But we already see how it +entails extinction; and how largely extinction has acted in the world’s +history, geology plainly declares. Natural selection, also, leads to +divergence of +character; for more living beings can be supported on the same area the +more they diverge in structure, habits, and constitution, of which we +see proof by looking at the inhabitants of any small spot or at +naturalised productions. Therefore during the modification of the +descendants of any one species, and during the incessant struggle of +all species to increase in numbers, the more diversified these +descendants become, the better will be their chance of succeeding in +the battle of life. Thus the small differences distinguishing varieties +of the same species, will steadily tend to increase till they come to +equal the greater differences between species of the same genus, or +even of distinct genera. + +We have seen that it is the common, the widely-diffused, and +widely-ranging species, belonging to the larger genera, which vary +most; and these will tend to transmit to their modified offspring that +superiority which now makes them dominant in their own countries. +Natural selection, as has just been remarked, leads to divergence of +character and to much extinction of the less improved and intermediate +forms of life. On these principles, I believe, the nature of the +affinities of all organic beings may be explained. It is a truly +wonderful fact—the wonder of which we are apt to overlook from +familiarity—that all animals and all plants throughout all time and +space should be related to each other in group subordinate to group, in +the manner which we everywhere behold—namely, varieties of the same +species most closely related together, species of the same genus less +closely and unequally related together, forming sections and +sub-genera, species of distinct genera much less closely related, and +genera related in different degrees, forming sub-families, families, +orders, sub-classes, and classes. The several subordinate groups in any +class cannot be +ranked in a single file, but seem rather to be clustered round points, +and these round other points, and so on in almost endless cycles. On +the view that each species has been independently created, I can see no +explanation of this great fact in the classification of all organic +beings; but, to the best of my judgment, it is explained through +inheritance and the complex action of natural selection, entailing +extinction and divergence of character, as we have seen illustrated in +the diagram. + +The affinities of all the beings of the same class have sometimes been +represented by a great tree. I believe this simile largely speaks the +truth. The green and budding twigs may represent existing species; and +those produced during each former year may represent the long +succession of extinct species. At each period of growth all the growing +twigs have tried to branch out on all sides, and to overtop and kill +the surrounding twigs and branches, in the same manner as species and +groups of species have tried to overmaster other species in the great +battle for life. The limbs divided into great branches, and these into +lesser and lesser branches, were themselves once, when the tree was +small, budding twigs; and this connexion of the former and present buds +by ramifying branches may well represent the classification of all +extinct and living species in groups subordinate to groups. Of the many +twigs which flourished when the tree was a mere bush, only two or +three, now grown into great branches, yet survive and bear all the +other branches; so with the species which lived during long-past +geological periods, very few now have living and modified descendants. +From the first growth of the tree, many a limb and branch has decayed +and dropped off; and these lost branches of various sizes may represent +those whole orders, families, and genera which have now no living +representatives, and +which are known to us only from having been found in a fossil state. As +we here and there see a thin straggling branch springing from a fork +low down in a tree, and which by some chance has been favoured and is +still alive on its summit, so we occasionally see an animal like the +Ornithorhynchus or Lepidosiren, which in some small degree connects by +its affinities two large branches of life, and which has apparently +been saved from fatal competition by having inhabited a protected +station. As buds give rise by growth to fresh buds, and these, if +vigorous, branch out and overtop on all sides many a feebler branch, so +by generation I believe it has been with the great Tree of Life, which +fills with its dead and broken branches the crust of the earth, and +covers the surface with its ever branching and beautiful ramifications. + + + + +CHAPTER V. +LAWS OF VARIATION. + + +Effects of external conditions. Use and disuse, combined with natural +selection; organs of flight and of vision. Acclimatisation. Correlation +of growth. Compensation and economy of growth. False correlations. +Multiple, rudimentary, and lowly organised structures variable. Parts +developed in an unusual manner are highly variable: specific characters +more variable than generic: secondary sexual characters variable. +Species of the same genus vary in an analogous manner. Reversions to +long lost characters. Summary. + + +I have hitherto sometimes spoken as if the variations—so common and +multiform in organic beings under domestication, and in a lesser degree +in those in a state of nature—had been due to chance. This, of course, +is a wholly incorrect expression, but it serves to acknowledge plainly +our ignorance of the cause of each particular variation. Some authors +believe it to be as much the function of the reproductive system to +produce individual differences, or very slight deviations of structure, +as to make the child like its parents. But the much greater +variability, as well as the greater frequency of monstrosities, under +domestication or cultivation, than under nature, leads me to believe +that deviations of structure are in some way due to the nature of the +conditions of life, to which the parents and their more remote +ancestors have been exposed during several generations. I have remarked +in the first chapter—but a long catalogue of facts which cannot be here +given would be necessary to show the truth of the remark—that the +reproductive system is eminently susceptible to changes in the +conditions of life; and to +this system being functionally disturbed in the parents, I chiefly +attribute the varying or plastic condition of the offspring. The male +and female sexual elements seem to be affected before that union takes +place which is to form a new being. In the case of “sporting” plants, +the bud, which in its earliest condition does not apparently differ +essentially from an ovule, is alone affected. But why, because the +reproductive system is disturbed, this or that part should vary more or +less, we are profoundly ignorant. Nevertheless, we can here and there +dimly catch a faint ray of light, and we may feel sure that there must +be some cause for each deviation of structure, however slight. + +How much direct effect difference of climate, food, etc., produces on +any being is extremely doubtful. My impression is, that the effect is +extremely small in the case of animals, but perhaps rather more in that +of plants. We may, at least, safely conclude that such influences +cannot have produced the many striking and complex co-adaptations of +structure between one organic being and another, which we see +everywhere throughout nature. Some little influence may be attributed +to climate, food, etc.: thus, E. Forbes speaks confidently that shells +at their southern limit, and when living in shallow water, are more +brightly coloured than those of the same species further north or from +greater depths. Gould believes that birds of the same species are more +brightly coloured under a clear atmosphere, than when living on islands +or near the coast. So with insects, Wollaston is convinced that +residence near the sea affects their colours. Moquin-Tandon gives a +list of plants which when growing near the sea-shore have their leaves +in some degree fleshy, though not elsewhere fleshy. Several other such +cases could be given. + +The fact of varieties of one species, when they range +into the zone of habitation of other species, often acquiring in a very +slight degree some of the characters of such species, accords with our +view that species of all kinds are only well-marked and permanent +varieties. Thus the species of shells which are confined to tropical +and shallow seas are generally brighter-coloured than those confined to +cold and deeper seas. The birds which are confined to continents are, +according to Mr. Gould, brighter-coloured than those of islands. The +insect-species confined to sea-coasts, as every collector knows, are +often brassy or lurid. Plants which live exclusively on the sea-side +are very apt to have fleshy leaves. He who believes in the creation of +each species, will have to say that this shell, for instance, was +created with bright colours for a warm sea; but that this other shell +became bright-coloured by variation when it ranged into warmer or +shallower waters. + +When a variation is of the slightest use to a being, we cannot tell how +much of it to attribute to the accumulative action of natural +selection, and how much to the conditions of life. Thus, it is well +known to furriers that animals of the same species have thicker and +better fur the more severe the climate is under which they have lived; +but who can tell how much of this difference may be due to the +warmest-clad individuals having been favoured and preserved during many +generations, and how much to the direct action of the severe climate? +for it would appear that climate has some direct action on the hair of +our domestic quadrupeds. + +Instances could be given of the same variety being produced under +conditions of life as different as can well be conceived; and, on the +other hand, of different varieties being produced from the same species +under the same conditions. Such facts show how indirectly the +conditions of life must act. Again, innumerable instances are known to +every naturalist of species keeping true, or not varying at all, +although living under the most opposite climates. Such considerations +as these incline me to lay very little weight on the direct action of +the conditions of life. Indirectly, as already remarked, they seem to +play an important part in affecting the reproductive system, and in +thus inducing variability; and natural selection will then accumulate +all profitable variations, however slight, until they become plainly +developed and appreciable by us. + +_Effects of Use and Disuse_.—From the facts alluded to in the first +chapter, I think there can be little doubt that use in our domestic +animals strengthens and enlarges certain parts, and disuse diminishes +them; and that such modifications are inherited. Under free nature, we +can have no standard of comparison, by which to judge of the effects of +long-continued use or disuse, for we know not the parent-forms; but +many animals have structures which can be explained by the effects of +disuse. As Professor Owen has remarked, there is no greater anomaly in +nature than a bird that cannot fly; yet there are several in this +state. The logger-headed duck of South America can only flap along the +surface of the water, and has its wings in nearly the same condition as +the domestic Aylesbury duck. As the larger ground-feeding birds seldom +take flight except to escape danger, I believe that the nearly wingless +condition of several birds, which now inhabit or have lately inhabited +several oceanic islands, tenanted by no beast of prey, has been caused +by disuse. The ostrich indeed inhabits continents and is exposed to +danger from which it cannot escape by flight, but by kicking it can +defend itself from enemies, as well as any of the smaller +quadrupeds. We may imagine that the early progenitor of the ostrich had +habits like those of a bustard, and that as natural selection increased +in successive generations the size and weight of its body, its legs +were used more, and its wings less, until they became incapable of +flight. + +Kirby has remarked (and I have observed the same fact) that the +anterior tarsi, or feet, of many male dung-feeding beetles are very +often broken off; he examined seventeen specimens in his own +collection, and not one had even a relic left. In the Onites apelles +the tarsi are so habitually lost, that the insect has been described as +not having them. In some other genera they are present, but in a +rudimentary condition. In the Ateuchus or sacred beetle of the +Egyptians, they are totally deficient. There is not sufficient evidence +to induce us to believe that mutilations are ever inherited; and I +should prefer explaining the entire absence of the anterior tarsi in +Ateuchus, and their rudimentary condition in some other genera, by the +long-continued effects of disuse in their progenitors; for as the tarsi +are almost always lost in many dung-feeding beetles, they must be lost +early in life, and therefore cannot be much used by these insects. + +In some cases we might easily put down to disuse modifications of +structure which are wholly, or mainly, due to natural selection. Mr. +Wollaston has discovered the remarkable fact that 200 beetles, out of +the 550 species inhabiting Madeira, are so far deficient in wings that +they cannot fly; and that of the twenty-nine endemic genera, no less +than twenty-three genera have all their species in this condition! +Several facts, namely, that beetles in many parts of the world are very +frequently blown to sea and perish; that the beetles 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 Dezertas than in Madeira +itself; and especially the extraordinary fact, so strongly insisted on +by Mr. Wollaston, of the almost entire absence of certain large groups +of beetles, elsewhere excessively numerous, and which groups have +habits of life almost necessitating frequent flight;—these several +considerations have made me believe that the wingless condition of so +many Madeira beetles is mainly due to the action of natural selection, +but combined probably with disuse. For during thousands of successive +generations each individual beetle which flew least, either from its +wings having been ever so little less perfectly developed or from +indolent habit, will have had the best chance of surviving from not +being blown out to sea; and, on the other hand, those beetles which +most readily took to flight will oftenest have been blown to sea and +thus have been destroyed. + +The insects in Madeira which are not ground-feeders, and which, as the +flower-feeding coleoptera and lepidoptera, must habitually use their +wings to gain their subsistence, have, as Mr. Wollaston suspects, their +wings not at all reduced, but even enlarged. This is quite compatible +with the action of natural selection. For when a new insect first +arrived on the island, the tendency of natural selection to enlarge or +to reduce the wings, would depend on whether a greater number of +individuals were saved by successfully battling with the winds, or by +giving up the attempt and rarely or never flying. As with mariners +shipwrecked near a coast, it would have been better for the good +swimmers if they had been able to swim still further, whereas it would +have been better for the bad swimmers if they had not been able to swim +at all and had stuck to the wreck. + + +The eyes of moles and of some burrowing rodents are rudimentary in +size, and in some cases are quite covered up by skin and fur. This +state of the eyes is probably due to gradual reduction from disuse, but +aided perhaps by natural selection. In South America, a burrowing +rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its +habits than the mole; and I was assured by a Spaniard, who had often +caught them, that they were frequently blind; one which I kept alive +was certainly in this condition, the cause, as appeared on dissection, +having been inflammation of the nictitating membrane. As frequent +inflammation of the eyes must be injurious to any animal, and as eyes +are certainly not indispensable to animals with subterranean habits, a +reduction in their size with the adhesion of the eyelids and growth of +fur over them, might in such case be an advantage; and if so, natural +selection would constantly aid the effects of disuse. + +It is well known that several animals, belonging to the most different +classes, which inhabit the caves of Styria and of Kentucky, are blind. +In some of the crabs the foot-stalk for the eye remains, though the eye +is gone; the stand for the telescope is there, though the telescope +with its glasses has been lost. As it is difficult to imagine that +eyes, though useless, could be in any way injurious to animals living +in darkness, I attribute their loss wholly to disuse. In one of the +blind animals, namely, the cave-rat, the eyes are of immense size; and +Professor Silliman thought that it regained, after living some days in +the light, some slight power of vision. In the same manner as in +Madeira the wings of some of the insects have been enlarged, and the +wings of others have been reduced by natural selection aided by use and +disuse, so in the case of the cave-rat natural selection seems to have +struggled with the loss of light and +to have increased the size of the eyes; whereas with all the other +inhabitants of the caves, disuse by itself seems to have done its work. + +It is difficult to imagine conditions of life more similar than deep +limestone caverns under a nearly similar climate; so that on the common +view of the blind animals having been separately created for the +American and European caverns, close similarity in their organisation +and affinities might have been expected; but, as Schiödte and others +have remarked, this is not the case, and the cave-insects of the two +continents are not more closely allied than might have been anticipated +from the general resemblance of the other inhabitants of North America +and Europe. On my view we must suppose that American animals, having +ordinary powers of vision, slowly migrated by successive generations +from the outer world into the deeper and deeper recesses of the +Kentucky caves, as did European animals into the caves of Europe. We +have some evidence of this gradation of habit; for, as Schiödte +remarks, “animals not far remote from ordinary forms, prepare the +transition from light to darkness. Next follow those that are +constructed for twilight; and, last of all, those destined for total +darkness.” By the time that an animal had reached, after numberless +generations, the deepest recesses, disuse will on this view have more +or less perfectly obliterated its eyes, and natural selection will +often have effected other changes, such as an increase in the length of +the antennæ or palpi, as a compensation for blindness. Notwithstanding +such modifications, we might expect still to see in the cave-animals of +America, affinities to the other inhabitants of that continent, and in +those of Europe, to the inhabitants of the European continent. And this +is the case with some of the American cave-animals, as I hear from +Professor Dana; and some of the European cave-insects are very closely +allied to those of the surrounding country. It would be most difficult +to give any rational explanation of the affinities of the blind +cave-animals to the other inhabitants of the two continents on the +ordinary view of their independent creation. That several of the +inhabitants of the caves of the Old and New Worlds should be closely +related, we might expect from the well-known relationship of most of +their other productions. Far from feeling any surprise that some of the +cave-animals should be very anomalous, as Agassiz has remarked in +regard to the blind fish, the Amblyopsis, and as is the case with the +blind Proteus with reference to the reptiles of Europe, I am only +surprised that more wrecks of ancient life have not been preserved, +owing to the less severe competition to which the inhabitants of these +dark abodes will probably have been exposed. + +_Acclimatisation_.—Habit is hereditary with plants, as in the period of +flowering, in the amount of rain requisite for seeds to germinate, in +the time of sleep, etc., and this leads me to say a few words on +acclimatisation. As it is extremely common for species of the same +genus to inhabit very hot and very cold countries, and as I believe +that all the species of the same genus have descended from a single +parent, if this view be correct, acclimatisation must be readily +effected during long-continued descent. It is notorious that each +species is adapted to the climate of its own home: species from an +arctic or even from a temperate region cannot endure a tropical +climate, or conversely. So again, many succulent plants cannot endure a +damp climate. But the degree of adaptation of species to the climates +under which they live is often overrated. +We may infer this from our frequent inability to predict whether or not +an imported plant will endure our climate, and from the number of +plants and animals brought from warmer countries which here enjoy good +health. We have reason to believe that species in a state of nature are +limited in their ranges by the competition of other organic beings +quite as much as, or more than, by adaptation to particular climates. +But whether or not the adaptation be generally very close, we have +evidence, in the case of some few plants, of their becoming, to a +certain extent, naturally habituated to different temperatures, or +becoming acclimatised: thus the pines and rhododendrons, raised from +seed collected by Dr. Hooker from trees growing at different heights on +the Himalaya, were found in this country to possess different +constitutional powers of resisting cold. Mr. Thwaites informs me that +he has observed similar facts in Ceylon, and analogous observations +have been made by Mr. H. C. Watson on European species of plants +brought from the Azores to England. In regard to animals, several +authentic cases could be given of species within historical times +having largely extended their range from warmer to cooler latitudes, +and conversely; but we do not positively know that these animals were +strictly adapted to their native climate, but in all ordinary cases we +assume such to be the case; nor do we know that they have subsequently +become acclimatised to their new homes. + +As I believe that our domestic animals were originally chosen by +uncivilised man because they were useful and bred readily under +confinement, and not because they were subsequently found capable of +far-extended transportation, I think the common and extraordinary +capacity in our domestic animals of not only withstanding the most +different climates but of being perfectly +fertile (a far severer test) under them, may be used as an argument +that a large proportion of other animals, now in a state of nature, +could easily be brought to bear widely different climates. We must not, +however, push the foregoing argument too far, on account of the +probable origin of some of our domestic animals from several wild +stocks: the blood, for instance, of a tropical and arctic wolf or wild +dog may perhaps be mingled in our domestic breeds. The rat and mouse +cannot be considered as domestic animals, but they have been +transported by man to many parts of the world, and now have a far wider +range than any other rodent, living free under the cold climate of +Faroe in the north and of the Falklands in the south, and on many +islands in the torrid zones. Hence I am inclined to look at adaptation +to any special climate as a quality readily grafted on an innate wide +flexibility of constitution, which is common to most animals. On this +view, the capacity of enduring the most different climates by man +himself and by his domestic animals, and such facts as that former +species of the elephant and rhinoceros were capable of enduring a +glacial climate, whereas the living species are now all tropical or +sub-tropical in their habits, ought not to be looked at as anomalies, +but merely as examples of a very common flexibility of constitution, +brought, under peculiar circumstances, into play. + +How much of the acclimatisation of species to any peculiar climate is +due to mere habit, and how much to the natural selection of varieties +having different innate constitutions, and how much to both means +combined, is a very obscure question. That habit or custom has some +influence I must believe, both from analogy, and from the incessant +advice given in agricultural works, even in the ancient Encyclopædias +of China, to be very cautious +in transposing animals from one district to another; for it is not +likely that man should have succeeded in selecting so many breeds and +sub-breeds with constitutions specially fitted for their own districts: +the result must, I think, be due to habit. On the other hand, I can see +no reason to doubt that natural selection will continually tend to +preserve those individuals which are born with constitutions best +adapted to their native countries. In treatises on many kinds of +cultivated plants, certain varieties are said to withstand certain +climates better than others: this is very strikingly shown in works on +fruit trees published in the United States, in which certain varieties +are habitually recommended for the northern, and others for the +southern States; and as most of these varieties are of recent origin, +they cannot owe their constitutional differences to habit. The case of +the Jerusalem artichoke, which is never propagated by seed, and of +which consequently new varieties have not been produced, has even been +advanced—for it is now as tender as ever it was—as proving that +acclimatisation cannot be effected! The case, also, of the kidney-bean +has been often cited for a similar purpose, and with much greater +weight; but until some one will sow, during a score of generations, his +kidney-beans so early that a very large proportion are destroyed by +frost, and then collect seed from the few survivors, with care to +prevent accidental crosses, and then again get seed from these +seedlings, with the same precautions, the experiment cannot be said to +have been even tried. Nor let it be supposed that no differences in the +constitution of seedling kidney-beans ever appear, for an account has +been published how much more hardy some seedlings appeared to be than +others. + +On the whole, I think we may conclude that habit, +use, and disuse, have, in some cases, played a considerable part in the +modification of the constitution, and of the structure of various +organs; but that the effects of use and disuse have often been largely +combined with, and sometimes overmastered by, the natural selection of +innate differences. + +_Correlation of Growth_.—I mean by this expression that the whole +organisation is so tied together during its growth and development, +that when slight variations in any one part occur, and are accumulated +through natural selection, other parts become modified. This is a very +important subject, most imperfectly understood. The most obvious case +is, that modifications accumulated solely for the good of the young or +larva, will, it may safely be concluded, affect the structure of the +adult; in the same manner as any malconformation affecting the early +embryo, seriously affects the whole organisation of the adult. The +several parts of the body which are homologous, and which, at an early +embryonic period, are alike, seem liable to vary in an allied manner: +we see this in the right and left sides of the body varying in the same +manner; in the front and hind legs, and even in the jaws and limbs, +varying together, for the lower jaw is believed to be homologous with +the limbs. These tendencies, I do not doubt, may be mastered more or +less completely by natural selection: thus a family of stags once +existed with an antler only on one side; and if this had been of any +great use to the breed it might probably have been rendered permanent +by natural selection. + +Homologous parts, as has been remarked by some authors, tend to cohere; +this is often seen in monstrous plants; and nothing is more common than +the union of homologous parts in normal structures, as the union of +the petals of the corolla into a tube. Hard parts seem to affect the +form of adjoining soft parts; it is believed by some authors that the +diversity in the shape of the pelvis in birds causes the remarkable +diversity in the shape of their kidneys. Others believe that the shape +of the pelvis in the human mother influences by pressure the shape of +the head of the child. In snakes, according to Schlegel, the shape of +the body and the manner of swallowing determine the position of several +of the most important viscera. + +The nature of the bond of correlation is very frequently quite obscure. +M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain +malconformations very frequently, and that others rarely coexist, +without our being able to assign any reason. What can be more singular +than the relation between blue eyes and deafness in cats, and the +tortoise-shell colour with the female sex; the feathered feet and skin +between the outer toes in pigeons, and the presence of more or less +down on the young birds when first hatched, with the future colour of +their plumage; or, again, the relation between the hair and teeth in +the naked Turkish dog, though here probably homology comes into play? +With respect to this latter case of correlation, I think it can hardly +be accidental, that if we pick out the two orders of mammalia which are +most abnormal in their dermal coverings, viz. Cetacea (whales) and +Edentata (armadilloes, scaly ant-eaters, etc.), that these are likewise +the most abnormal in their teeth. + +I know of no case better adapted to show the importance of the laws of +correlation in modifying important structures, independently of utility +and, therefore, of natural selection, than that of the difference +between the outer and inner flowers in some Compositous and +Umbelliferous plants. Every one knows the +difference in the ray and central florets of, for instance, the daisy, +and this difference is often accompanied with the abortion of parts of +the flower. But, in some Compositous plants, the seeds also differ in +shape and sculpture; and even the ovary itself, with its accessory +parts, differs, as has been described by Cassini. These differences +have been attributed by some authors to pressure, and the shape of the +seeds in the ray-florets in some Compositæ countenances this idea; but, +in the case of the corolla of the Umbelliferæ, it is by no means, as +Dr. Hooker informs me, in species with the densest heads that the inner +and outer flowers most frequently differ. It might have been thought +that the development of the ray-petals by drawing nourishment from +certain other parts of the flower had caused their abortion; but in +some Compositæ there is a difference in the seeds of the outer and +inner florets without any difference in the corolla. Possibly, these +several differences may be connected with some difference in the flow +of nutriment towards the central and external flowers: we know, at +least, that in irregular flowers, those nearest to the axis are +oftenest subject to peloria, and become regular. I may add, as an +instance of this, and of a striking case of correlation, that I have +recently observed in some garden pelargoniums, that the central flower +of the truss often loses the patches of darker colour in the two upper +petals; and that when this occurs, the adherent nectary is quite +aborted; when the colour is absent from only one of the two upper +petals, the nectary is only much shortened. + +With respect to the difference in the corolla of the central and +exterior flowers of a head or umbel, I do not feel at all sure that C. +C. Sprengel’s idea that the ray-florets serve to attract insects, whose +agency is highly advantageous in the fertilisation of plants of +these two orders, is so far-fetched, as it may at first appear: and if +it be advantageous, natural selection may have come into play. But in +regard to the differences both in the internal and external structure +of the seeds, which are not always correlated with any differences in +the flowers, it seems impossible that they can be in any way +advantageous to the plant: yet in the Umbelliferæ these differences are +of such apparent importance—the seeds being in some cases, according to +Tausch, orthospermous in the exterior flowers and coelospermous in the +central flowers,—that the elder De Candolle founded his main divisions +of the order on analogous differences. Hence we see that modifications +of structure, viewed by systematists as of high value, may be wholly +due to unknown laws of correlated growth, and without being, as far as +we can see, of the slightest service to the species. + +We may often falsely attribute to correlation of growth, structures +which are common to whole groups of species, and which in truth are +simply due to inheritance; for an ancient progenitor may have acquired +through natural selection some one modification in structure, and, +after thousands of generations, some other and independent +modification; and these two modifications, having been transmitted to a +whole group of descendants with diverse habits, would naturally be +thought to be correlated in some necessary manner. So, again, I do not +doubt that some apparent correlations, occurring throughout whole +orders, are entirely due to the manner alone in which natural selection +can act. For instance, Alph. De Candolle has remarked that winged seeds +are never found in fruits which do not open: I should explain the rule +by the fact that seeds could not gradually become winged through +natural selection, except in fruits which opened; so that the +individual plants producing +seeds which were a little better fitted to be wafted further, might get +an advantage over those producing seed less fitted for dispersal; and +this process could not possibly go on in fruit which did not open. + +The elder Geoffroy and Goethe propounded, at about the same period, +their law of compensation or balancement of growth; or, as Goethe +expressed it, “in order to spend on one side, nature is forced to +economise on the other side.” I think this holds true to a certain +extent with our domestic productions: if nourishment flows to one part +or organ in excess, it rarely flows, at least in excess, to another +part; thus it is difficult to get a cow to give much milk and to fatten +readily. The same varieties of the cabbage do not yield abundant and +nutritious foliage and a copious supply of oil-bearing seeds. When the +seeds in our fruits become atrophied, the fruit itself gains largely in +size and quality. In our poultry, a large tuft of feathers on the head +is generally accompanied by a diminished comb, and a large beard by +diminished wattles. With species in a state of nature it can hardly be +maintained that the law is of universal application; but many good +observers, more especially botanists, believe in its truth. I will not, +however, here give any instances, for I see hardly any way of +distinguishing between the effects, on the one hand, of a part being +largely developed through natural selection and another and adjoining +part being reduced by this same process or by disuse, and, on the other +hand, the actual withdrawal of nutriment from one part owing to the +excess of growth in another and adjoining part. + +I suspect, also, that some of the cases of compensation which have been +advanced, and likewise some other facts, may be merged under a more +general principle, namely, that natural selection is continually trying +to economise in every part of the organisation. If under +changed conditions of life a structure before useful becomes less +useful, any diminution, however slight, in its development, will be +seized on by natural selection, for it will profit the individual not +to have its nutriment wasted in building up an useless structure. I can +thus only understand a fact with which I was much struck when examining +cirripedes, and of which many other instances could be given: namely, +that when a cirripede is parasitic within another and is thus +protected, it loses more or less completely its own shell or carapace. +This is the case with the male Ibla, and in a truly extraordinary +manner with the Proteolepas: for the carapace in all other cirripedes +consists of the three highly-important anterior segments of the head +enormously developed, and furnished with great nerves and muscles; but +in the parasitic and protected Proteolepas, the whole anterior part of +the head is reduced to the merest rudiment attached to the bases of the +prehensile antennæ. Now the saving of a large and complex structure, +when rendered superfluous by the parasitic habits of the Proteolepas, +though effected by slow steps, would be a decided advantage to each +successive individual of the species; for in the struggle for life to +which every animal is exposed, each individual Proteolepas would have a +better chance of supporting itself, by less nutriment being wasted in +developing a structure now become useless. + +Thus, as I believe, natural selection will always succeed in the long +run in reducing and saving every part of the organisation, as soon as +it is rendered superfluous, without by any means causing some other +part to be largely developed in a corresponding degree. And, +conversely, that natural selection may perfectly well succeed in +largely developing any organ, without requiring as a necessary +compensation the reduction of some adjoining part. + + +It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in +varieties and in species, that when any part or organ is repeated many +times in the structure of the same individual (as the vertebræ in +snakes, and the stamens in polyandrous flowers) the number is variable; +whereas the number of the same part or organ, when it occurs in lesser +numbers, is constant. The same author and some botanists have further +remarked that multiple parts are also very liable to variation in +structure. Inasmuch as this “vegetative repetition,” to use Professor +Owen’s expression, seems to be a sign of low organisation; the +foregoing remark seems connected with the very general opinion of +naturalists, that beings low in the scale of nature are more variable +than those which are higher. I presume that lowness in this case means +that the several parts of the organisation have been but little +specialised for particular functions; and as long as the same part has +to perform diversified work, we can perhaps see why it should remain +variable, that is, why natural selection should have preserved or +rejected each little deviation of form less carefully than when the +part has to serve for one special purpose alone. In the same way that a +knife which has to cut all sorts of things may be of almost any shape; +whilst a tool for some particular object had better be of some +particular shape. Natural selection, it should never be forgotten, can +act on each part of each being, solely through and for its advantage. + +Rudimentary parts, it has been stated by some authors, and I believe +with truth, are apt to be highly variable. We shall have to recur to +the general subject of rudimentary and aborted organs; and I will here +only add that their variability seems to be owing to their uselessness, +and therefore to natural selection having no power to check deviations +in their structure. Thus +rudimentary parts are left to the free play of the various laws of +growth, to the effects of long-continued disuse, and to the tendency to +reversion. + +_A part developed in any species in an extraordinary degree or manner, +in comparison with the same part in allied species, tends to be highly +variable_.—Several years ago I was much struck with a remark, nearly to +the above effect, published by Mr. Waterhouse. I infer also from an +observation made by Professor Owen, with respect to the length of the +arms of the ourang-outang, that he has come to a nearly similar +conclusion. It is hopeless to attempt to convince any one of the truth +of this proposition without giving the long array of facts which I have +collected, and which cannot possibly be here introduced. I can only +state my conviction that it is a rule of high generality. I am aware of +several causes of error, but I hope that I have made due allowance for +them. It should be understood that the rule by no means applies to any +part, however unusually developed, unless it be unusually developed in +comparison with the same part in closely allied species. Thus, the +bat’s wing is a most abnormal structure in the class mammalia; but the +rule would not here apply, because there is a whole group of bats +having wings; it would apply only if some one species of bat had its +wings developed in some remarkable manner in comparison with the other +species of the same genus. The rule applies very strongly in the case +of secondary sexual characters, when displayed in any unusual manner. +The term, secondary sexual characters, used by Hunter, applies to +characters which are attached to one sex, but are not directly +connected with the act of reproduction. The rule applies to males and +females; but as females more rarely offer remarkable secondary sexual +characters, it applies +more rarely to them. The rule being so plainly applicable in the case +of secondary sexual characters, may be due to the great variability of +these characters, whether or not displayed in any unusual manner—of +which fact I think there can be little doubt. But that our rule is not +confined to secondary sexual characters is clearly shown in the case of +hermaphrodite cirripedes; and I may here add, that I particularly +attended to Mr. Waterhouse’s remark, whilst investigating this Order, +and I am fully convinced that the rule almost invariably holds good +with cirripedes. I shall, in my future work, give a list of the more +remarkable cases; I will here only briefly give one, as it illustrates +the rule in its largest application. The opercular valves of sessile +cirripedes (rock barnacles) are, in every sense of the word, very +important structures, and they differ extremely little even in +different genera; but in the several species of one genus, Pyrgoma, +these valves present a marvellous amount of diversification: the +homologous valves in the different species being sometimes wholly +unlike in shape; and the amount of variation in the individuals of +several of the species is so great, that it is no exaggeration to state +that the varieties differ more from each other in the characters of +these important valves than do other species of distinct genera. + +As birds within the same country vary in a remarkably small degree, I +have particularly attended to them, and the rule seems to me certainly +to hold good in this class. I cannot make out that it applies to +plants, and this would seriously have shaken my belief in its truth, +had not the great variability in plants made it particularly difficult +to compare their relative degrees of variability. + +When we see any part or organ developed in a remarkable degree or +manner in any species, the fair +presumption is that it is of high importance to that species; +nevertheless the part in this case is eminently liable to variation. +Why should this be so? On the view that each species has been +independently created, with all its parts as we now see them, I can see +no explanation. But on the view that groups of species have descended +from other species, and have been modified through natural selection, I +think we can obtain some light. In our domestic animals, if any part, +or the whole animal, be neglected and no selection be applied, that +part (for instance, the comb in the Dorking fowl) or the whole breed +will cease to have a nearly uniform character. The breed will then be +said to have degenerated. In rudimentary organs, and in those which +have been but little specialised for any particular purpose, and +perhaps in polymorphic groups, we see a nearly parallel natural case; +for in such cases natural selection either has not or cannot come into +full play, and thus the organisation is left in a fluctuating +condition. But what here more especially concerns us is, that in our +domestic animals those points, which at the present time are undergoing +rapid change by continued selection, are also eminently liable to +variation. Look at the breeds of the pigeon; see what a prodigious +amount of difference there is in the beak of the different tumblers, in +the beak and wattle of the different carriers, in the carriage and tail +of our fantails, etc., these being the points now mainly attended to by +English fanciers. Even in the sub-breeds, as in the short-faced +tumbler, it is notoriously difficult to breed them nearly to +perfection, and frequently individuals are born which depart widely +from the standard. There may be truly said to be a constant struggle +going on between, on the one hand, the tendency to reversion to a less +modified state, as well as an innate tendency to further +variability of all kinds, and, on the other hand, the power of steady +selection to keep the breed true. In the long run selection gains the +day, and we do not expect to fail so far as to breed a bird as coarse +as a common tumbler from a good short-faced strain. But as long as +selection is rapidly going on, there may always be expected to be much +variability in the structure undergoing modification. It further +deserves notice that these variable characters, produced by man’s +selection, sometimes become attached, from causes quite unknown to us, +more to one sex than to the other, generally to the male sex, as with +the wattle of carriers and the enlarged crop of pouters. + +Now let us turn to nature. When a part has been developed in an +extraordinary manner in any one species, compared with the other +species of the same genus, we may conclude that this part has undergone +an extraordinary amount of modification, since the period when the +species branched off from the common progenitor of the genus. This +period will seldom be remote in any extreme degree, as species very +rarely endure for more than one geological period. An extraordinary +amount of modification implies an unusually large and long-continued +amount of variability, which has continually been accumulated by +natural selection for the benefit of the species. But as the +variability of the extraordinarily-developed part or organ has been so +great and long-continued within a period not excessively remote, we +might, as a general rule, expect still to find more variability in such +parts than in other parts of the organisation, which have remained for +a much longer period nearly constant. And this, I am convinced, is the +case. That the struggle between natural selection on the one hand, and +the tendency to reversion and variability on the other hand, will in +the +course of time cease; and that the most abnormally developed organs may +be made constant, I can see no reason to doubt. Hence when an organ, +however abnormal it may be, has been transmitted in approximately the +same condition to many modified descendants, as in the case of the wing +of the bat, it must have existed, according to my theory, for an +immense period in nearly the same state; and thus it comes to be no +more variable than any other structure. It is only in those cases in +which the modification has been comparatively recent and +extraordinarily great that we ought to find the _generative +variability_, as it may be called, still present in a high degree. For +in this case the variability will seldom as yet have been fixed by the +continued selection of the individuals varying in the required manner +and degree, and by the continued rejection of those tending to revert +to a former and less modified condition. + +The principle included in these remarks may be extended. It is +notorious that specific characters are more variable than generic. To +explain by a simple example what is meant. If some species in a large +genus of plants had blue flowers and some had red, the colour would be +only a specific character, and no one would be surprised at one of the +blue species varying into red, or conversely; but if all the species +had blue flowers, the colour would become a generic character, and its +variation would be a more unusual circumstance. I have chosen this +example because an explanation is not in this case applicable, which +most naturalists would advance, namely, that specific characters are +more variable than generic, because they are taken from parts of less +physiological importance than those commonly used for classing genera. +I believe this explanation is partly, yet only indirectly, true; I +shall, however, have to return +to this subject in our chapter on Classification. It would be almost +superfluous to adduce evidence in support of the above statement, that +specific characters are more variable than generic; but I have +repeatedly noticed in works on natural history, that when an author has +remarked with surprise that some _important_ organ or part, which is +generally very constant throughout large groups of species, has +_differed_ considerably in closely-allied species, that it has, also, +been _variable_ in the individuals of some of the species. And this +fact shows that a character, which is generally of generic value, when +it sinks in value and becomes only of specific value, often becomes +variable, though its physiological importance may remain the same. +Something of the same kind applies to monstrosities: at least Is. +Geoffroy St. Hilaire seems to entertain no doubt, that the more an +organ normally differs in the different species of the same group, the +more subject it is to individual anomalies. + +On the ordinary view of each species having been independently created, +why should that part of the structure, which differs from the same part +in other independently-created species of the same genus, be more +variable than those parts which are closely alike in the several +species? I do not see that any explanation can be given. But on the +view of species being only strongly marked and fixed varieties, we +might surely expect to find them still often continuing to vary in +those parts of their structure which have varied within a moderately +recent period, and which have thus come to differ. Or to state the case +in another manner:—the points in which all the species of a genus +resemble each other, and in which they differ from the species of some +other genus, are called generic characters; and these characters in +common I attribute to inheritance from a common +progenitor, for it can rarely have happened that natural selection will +have modified several species, fitted to more or less widely-different +habits, in exactly the same manner: and as these so-called generic +characters have been inherited from a remote period, since that period +when the species first branched off from their common progenitor, and +subsequently have not varied or come to differ in any degree, or only +in a slight degree, it is not probable that they should vary at the +present day. On the other hand, the points in which species differ from +other species of the same genus, are called specific characters; and as +these specific characters have varied and come to differ within the +period of the branching off of the species from a common progenitor, it +is probable that they should still often be in some degree variable,—at +least more variable than those parts of the organisation which have for +a very long period remained constant. + +In connexion with the present subject, I will make only two other +remarks. I think it will be admitted, without my entering on details, +that secondary sexual characters are very variable; I think it also +will be admitted that species of the same group differ from each other +more widely in their secondary sexual characters, than in other parts +of their organisation; compare, for instance, the amount of difference +between the males of gallinaceous birds, in which secondary sexual +characters are strongly displayed, with the amount of difference +between their females; and the truth of this proposition will be +granted. The cause of the original variability of secondary sexual +characters is not manifest; but we can see why these characters should +not have been rendered as constant and uniform as other parts of the +organisation; for secondary sexual characters have been accumulated by +sexual selection, which +is less rigid in its action than ordinary selection, as it does not +entail death, but only gives fewer offspring to the less favoured +males. Whatever the cause may be of the variability of secondary sexual +characters, as they are highly variable, sexual selection will have had +a wide scope for action, and may thus readily have succeeded in giving +to the species of the same group a greater amount of difference in +their sexual characters, than in other parts of their structure. + +It is a remarkable fact, that the secondary sexual differences between +the two sexes of the same species are generally displayed in the very +same parts of the organisation in which the different species of the +same genus differ from each other. Of this fact I will give in +illustration two instances, the first which happen to stand on my list; +and as the differences in these cases are of a very unusual nature, the +relation can hardly be accidental. The same number of joints in the +tarsi is a character generally common to very large groups of beetles, +but in the Engidæ, as Westwood has remarked, the number varies greatly; +and the number likewise differs in the two sexes of the same species: +again in fossorial hymenoptera, the manner of neuration of the wings is +a character of the highest importance, because common to large groups; +but in certain genera the neuration differs in the different species, +and likewise in the two sexes of the same species. This relation has a +clear meaning on my view of the subject: I look at all the species of +the same genus as having as certainly descended from the same +progenitor, as have the two sexes of any one of the species. +Consequently, whatever part of the structure of the common progenitor, +or of its early descendants, became variable; variations of this part +would it is highly probable, be taken advantage of by natural and +sexual selection, in +order to fit the several species to their several places in the economy +of nature, and likewise to fit the two sexes of the same species to +each other, or to fit the males and females to different habits of +life, or the males to struggle with other males for the possession of +the females. + +Finally, then, I conclude that the greater variability of specific +characters, or those which distinguish species from species, than of +generic characters, or those which the species possess in common;—that +the frequent extreme variability of any part which is developed in a +species in an extraordinary manner in comparison with the same part in +its congeners; and the not great degree of variability in a part, +however extraordinarily it may be developed, if it be common to a whole +group of species;—that the great variability of secondary sexual +characters, and the great amount of difference in these same characters +between closely allied species;—that secondary sexual and ordinary +specific differences are generally displayed in the same parts of the +organisation,—are all principles closely connected together. All being +mainly due to the species of the same group having descended from a +common progenitor, from whom they have inherited much in common,—to +parts which have recently and largely varied being more likely still to +go on varying than parts which have long been inherited and have not +varied,—to natural selection having more or less completely, according +to the lapse of time, overmastered the tendency to reversion and to +further variability,—to sexual selection being less rigid than ordinary +selection,—and to variations in the same parts having been accumulated +by natural and sexual selection, and thus adapted for secondary sexual, +and for ordinary specific purposes. + + +_Distinct species present analogous variations; and a variety of one +species often assumes some of the characters of an allied species, or +reverts to some of the characters of an early progenitor_.—These +propositions will be most readily understood by looking to our domestic +races. The most distinct breeds of pigeons, in countries most widely +apart, present sub-varieties with reversed feathers on the head and +feathers on the feet,—characters not possessed by the aboriginal +rock-pigeon; these then are analogous variations in two or more +distinct races. The frequent presence of fourteen or even sixteen +tail-feathers in the pouter, may be considered as a variation +representing the normal structure of another race, the fantail. I +presume that no one will doubt that all such analogous variations are +due to the several races of the pigeon having inherited from a common +parent the same constitution and tendency to variation, when acted on +by similar unknown influences. In the vegetable kingdom we have a case +of analogous variation, in the enlarged stems, or roots as commonly +called, of the Swedish turnip and Ruta baga, plants which several +botanists rank as varieties produced by cultivation from a common +parent: if this be not so, the case will then be one of analogous +variation in two so-called distinct species; and to these a third may +be added, namely, the common turnip. According to the ordinary view of +each species having been independently created, we should have to +attribute this similarity in the enlarged stems of these three plants, +not to the vera causa of community of descent, and a consequent +tendency to vary in a like manner, but to three separate yet closely +related acts of creation. + +With pigeons, however, we have another case, namely, the occasional +appearance in all the breeds, of slaty-blue birds with two black bars +on the wings, a white +rump, a bar at the end of the tail, with the outer feathers externally +edged near their bases with white. As all these marks are +characteristic of the parent rock-pigeon, I presume that no one will +doubt that this is a case of reversion, and not of a new yet analogous +variation appearing in the several breeds. We may I think confidently +come to this conclusion, because, as we have seen, these coloured marks +are eminently liable to appear in the crossed offspring of two distinct +and differently coloured breeds; and in this case there is nothing in +the external conditions of life to cause the reappearance of the +slaty-blue, with the several marks, beyond the influence of the mere +act of crossing on the laws of inheritance. + +No doubt it is a very surprising fact that characters should reappear +after having been lost for many, perhaps for hundreds of generations. +But when a breed has been crossed only once by some other breed, the +offspring occasionally show a tendency to revert in character to the +foreign breed for many generations—some say, for a dozen or even a +score of generations. After twelve generations, the proportion of +blood, to use a common expression, of any one ancestor, is only 1 in +2048; and yet, as we see, it is generally believed that a tendency to +reversion is retained by this very small proportion of foreign blood. +In a breed which has not been crossed, but in which _both_ parents have +lost some character which their progenitor possessed, the tendency, +whether strong or weak, to reproduce the lost character might be, as +was formerly remarked, for all that we can see to the contrary, +transmitted for almost any number of generations. When a character +which has been lost in a breed, reappears after a great number of +generations, the most probable hypothesis is, not that the offspring +suddenly takes after an ancestor some hundred generations +distant, but that in each successive generation there has been a +tendency to reproduce the character in question, which at last, under +unknown favourable conditions, gains an ascendancy. For instance, it is +probable that in each generation of the barb-pigeon, which produces +most rarely a blue and black-barred bird, there has been a tendency in +each generation in the plumage to assume this colour. This view is +hypothetical, but could be supported by some facts; and I can see no +more abstract improbability in a tendency to produce any character +being inherited for an endless number of generations, than in quite +useless or rudimentary organs being, as we all know them to be, thus +inherited. Indeed, we may sometimes observe a mere tendency to produce +a rudiment inherited: for instance, in the common snapdragon +(Antirrhinum) a rudiment of a fifth stamen so often appears, that this +plant must have an inherited tendency to produce it. + +As all the species of the same genus are supposed, on my theory, to +have descended from a common parent, it might be expected that they +would occasionally vary in an analogous manner; so that a variety of +one species would resemble in some of its characters another species; +this other species being on my view only a well-marked and permanent +variety. But characters thus gained would probably be of an unimportant +nature, for the presence of all important characters will be governed +by natural selection, in accordance with the diverse habits of the +species, and will not be left to the mutual action of the conditions of +life and of a similar inherited constitution. It might further be +expected that the species of the same genus would occasionally exhibit +reversions to lost ancestral characters. As, however, we never know the +exact character of the common ancestor of a group, we could not +distinguish these two +cases: if, for instance, we did not know that the rock-pigeon was not +feather-footed or turn-crowned, we could not have told, whether these +characters in our domestic breeds were reversions or only analogous +variations; but we might have inferred that the blueness was a case of +reversion, from the number of the markings, which are correlated with +the blue tint, and which it does not appear probable would all appear +together from simple variation. More especially we might have inferred +this, from the blue colour and marks so often appearing when distinct +breeds of diverse colours are crossed. Hence, though under nature it +must generally be left doubtful, what cases are reversions to an +anciently existing character, and what are new but analogous +variations, yet we ought, on my theory, sometimes to find the varying +offspring of a species assuming characters (either from reversion or +from analogous variation) which already occur in some other members of +the same group. And this undoubtedly is the case in nature. + +A considerable part of the difficulty in recognising a variable species +in our systematic works, is due to its varieties mocking, as it were, +some of the other species of the same genus. A considerable catalogue, +also, could be given of forms intermediate between two other forms, +which themselves must be doubtfully ranked as either varieties or +species; and this shows, unless all these forms be considered as +independently created species, that the one in varying has assumed some +of the characters of the other, so as to produce the intermediate form. +But the best evidence is afforded by parts or organs of an important +and uniform nature occasionally varying so as to acquire, in some +degree, the character of the same part or organ in an allied species. I +have collected a long list of such cases; but +here, as before, I lie under a great disadvantage in not being able to +give them. I can only repeat that such cases certainly do occur, and +seem to me very remarkable. + +I will, however, give one curious and complex case, not indeed as +affecting any important character, but from occurring in several +species of the same genus, partly under domestication and partly under +nature. It is a case apparently of reversion. The ass not rarely has +very distinct transverse bars on its legs, like those on the legs of a +zebra: it has been asserted that these are plainest in the foal, and +from inquiries which I have made, I believe this to be true. It has +also been asserted that the stripe on each shoulder is sometimes +double. The shoulder stripe is certainly very variable in length and +outline. A white ass, but _not_ an albino, has been described without +either spinal or shoulder-stripe; and these stripes are sometimes very +obscure, or actually quite lost, in dark-coloured asses. The koulan of +Pallas is said to have been seen with a double shoulder-stripe. The +hemionus has no shoulder-stripe; but traces of it, as stated by Mr. +Blyth and others, occasionally appear: and I have been informed by +Colonel Poole that the foals of this species are generally striped on +the legs, and faintly on the shoulder. The quagga, though so plainly +barred like a zebra over the body, is without bars on the legs; but Dr. +Gray has figured one specimen with very distinct zebra-like bars on the +hocks. + +With respect to the horse, I have collected cases in England of the +spinal stripe in horses of the most distinct breeds, and of _all_ +colours; transverse bars on the legs are not rare in duns, mouse-duns, +and in one instance in a chestnut: a faint shoulder-stripe may +sometimes be seen in duns, and I have seen a trace in a +bay horse. My son made a careful examination and sketch for me of a dun +Belgian cart-horse with a double stripe on each shoulder and with +leg-stripes; and a man, whom I can implicitly trust, has examined for +me a small dun Welch pony with _three_ short parallel stripes on each +shoulder. + +In the north-west part of India the Kattywar breed of horses is so +generally striped, that, as I hear from Colonel Poole, who examined the +breed for the Indian Government, a horse without stripes is not +considered as purely-bred. The spine is always striped; the legs are +generally barred; and the shoulder-stripe, which is sometimes double +and sometimes treble, is common; the side of the face, moreover, is +sometimes striped. The stripes are plainest in the foal; and sometimes +quite disappear in old horses. Colonel Poole has seen both gray and bay +Kattywar horses striped when first foaled. I have, also, reason to +suspect, from information given me by Mr. W. W. Edwards, that with the +English race-horse the spinal stripe is much commoner in the foal than +in the full-grown animal. Without here entering on further details, I +may state that I have collected cases of leg and shoulder stripes in +horses of very different breeds, in various countries from Britain to +Eastern China; and from Norway in the north to the Malay Archipelago in +the south. In all parts of the world these stripes occur far oftenest +in duns and mouse-duns; by the term dun a large range of colour is +included, from one between brown and black to a close approach to +cream-colour. + +I am aware that Colonel Hamilton Smith, who has written on this +subject, believes that the several breeds of the horse have descended +from several aboriginal species—one of which, the dun, was striped; and +that the above-described appearances are all due to ancient +crosses with the dun stock. But I am not at all satisfied with this +theory, and should be loth to apply it to breeds so distinct as the +heavy Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race, +etc., inhabiting the most distant parts of the world. + +Now let us turn to the effects of crossing the several species of the +horse-genus. Rollin asserts, that the common mule from the ass and +horse is particularly apt to have bars on its legs. I once saw a mule +with its legs so much striped that any one at first would have thought +that it must have been the product of a zebra; and Mr. W. C. Martin, in +his excellent treatise on the horse, has given a figure of a similar +mule. In four coloured drawings, which I have seen, of hybrids between +the ass and zebra, the legs were much more plainly barred than the rest +of the body; and in one of them there was a double shoulder-stripe. In +Lord Moreton’s famous hybrid from a chestnut mare and male quagga, the +hybrid, and even the pure offspring subsequently produced from the mare +by a black Arabian sire, were much more plainly barred across the legs +than is even the pure quagga. Lastly, and this is another most +remarkable case, a hybrid has been figured by Dr. Gray (and he informs +me that he knows of a second case) from the ass and the hemionus; and +this hybrid, though the ass seldom has stripes on its legs and the +hemionus has none and has not even a shoulder-stripe, nevertheless had +all four legs barred, and had three short shoulder-stripes, like those +on the dun Welch pony, and even had some zebra-like stripes on the +sides of its face. With respect to this last fact, I was so convinced +that not even a stripe of colour appears from what would commonly be +called an accident, that I was led solely from the occurrence of the +face-stripes on this hybrid from the ass and hemionus, +to ask Colonel Poole whether such face-stripes ever occur in the +eminently striped Kattywar breed of horses, and was, as we have seen, +answered in the affirmative. + +What now are we to say to these several facts? We see several very +distinct species of the horse-genus becoming, by simple variation, +striped on the legs like a zebra, or striped on the shoulders like an +ass. In the horse we see this tendency strong whenever a dun tint +appears—a tint which approaches to that of the general colouring of the +other species of the genus. The appearance of the stripes is not +accompanied by any change of form or by any other new character. We see +this tendency to become striped most strongly displayed in hybrids from +between several of the most distinct species. Now observe the case of +the several breeds of pigeons: they are descended from a pigeon +(including two or three sub-species or geographical races) of a bluish +colour, with certain bars and other marks; and when any breed assumes +by simple variation a bluish tint, these bars and other marks +invariably reappear; but without any other change of form or character. +When the oldest and truest breeds of various colours are crossed, we +see a strong tendency for the blue tint and bars and marks to reappear +in the mongrels. I have stated that the most probable hypothesis to +account for the reappearance of very ancient characters, is—that there +is a _tendency_ in the young of each successive generation to produce +the long-lost character, and that this tendency, from unknown causes, +sometimes prevails. And we have just seen that in several species of +the horse-genus the stripes are either plainer or appear more commonly +in the young than in the old. Call the breeds of pigeons, some of which +have bred true for centuries, species; and how exactly parallel is the +case with that of the species of the horse-genus! +For myself, I venture confidently to look back thousands on thousands +of generations, and I see an animal striped like a zebra, but perhaps +otherwise very differently constructed, the common parent of our +domestic horse, whether or not it be descended from one or more wild +stocks, of the ass, the hemionus, quagga, and zebra. + +He who believes that each equine species was independently created, +will, I presume, assert that each species has been created with a +tendency to vary, both under nature and under domestication, in this +particular manner, so as often to become striped like other species of +the genus; and that each has been created with a strong tendency, when +crossed with species inhabiting distant quarters of the world, to +produce hybrids resembling in their stripes, not their own parents, but +other species of the genus. To admit this view is, as it seems to me, +to reject a real for an unreal, or at least for an unknown, cause. It +makes the works of God a mere mockery and deception; I would almost as +soon believe with the old and ignorant cosmogonists, that fossil shells +had never lived, but had been created in stone so as to mock the shells +now living on the sea-shore. + +_Summary_.—Our ignorance of the laws of variation is profound. Not in +one case out of a hundred can we pretend to assign any reason why this +or that part differs, more or less, from the same part in the parents. +But whenever we have the means of instituting a comparison, the same +laws appear to have acted in producing the lesser differences between +varieties of the same species, and the greater differences between +species of the same genus. The external conditions of life, as climate +and food, etc., seem to have induced some slight modifications. Habit +in producing constitutional differences, +and use in strengthening, and disuse in weakening and diminishing +organs, seem to have been more potent in their effects. Homologous +parts tend to vary in the same way, and homologous parts tend to +cohere. Modifications in hard parts and in external parts sometimes +affect softer and internal parts. When one part is largely developed, +perhaps it tends to draw nourishment from the adjoining parts; and +every part of the structure which can be saved without detriment to the +individual, will be saved. Changes of structure at an early age will +generally affect parts subsequently developed; and there are very many +other correlations of growth, the nature of which we are utterly unable +to understand. Multiple parts are variable in number and in structure, +perhaps arising from such parts not having been closely specialised to +any particular function, so that their modifications have not been +closely checked by natural selection. It is probably from this same +cause that organic beings low in the scale of nature are more variable +than those which have their whole organisation more specialised, and +are higher in the scale. Rudimentary organs, from being useless, will +be disregarded by natural selection, and hence probably are variable. +Specific characters—that is, the characters which have come to differ +since the several species of the same genus branched off from a common +parent—are more variable than generic characters, or those which have +long been inherited, and have not differed within this same period. In +these remarks we have referred to special parts or organs being still +variable, because they have recently varied and thus come to differ; +but we have also seen in the second Chapter that the same principle +applies to the whole individual; for in a district where many species +of any genus are found—that is, where there has been much former +variation and differentiation, or where the manufactory of new specific +forms has been actively at work—there, on an average, we now find most +varieties or incipient species. Secondary sexual characters are highly +variable, and such characters differ much in the species of the same +group. Variability in the same parts of the organisation has generally +been taken advantage of in giving secondary sexual differences to the +sexes of the same species, and specific differences to the several +species of the same genus. Any part or organ developed to an +extraordinary size or in an extraordinary manner, in comparison with +the same part or organ in the allied species, must have gone through an +extraordinary amount of modification since the genus arose; and thus we +can understand why it should often still be variable in a much higher +degree than other parts; for variation is a long-continued and slow +process, and natural selection will in such cases not as yet have had +time to overcome the tendency to further variability and to reversion +to a less modified state. But when a species with any +extraordinarily-developed organ has become the parent of many modified +descendants—which on my view must be a very slow process, requiring a +long lapse of time—in this case, natural selection may readily have +succeeded in giving a fixed character to the organ, in however +extraordinary a manner it may be developed. Species inheriting nearly +the same constitution from a common parent and exposed to similar +influences will naturally tend to present analogous variations, and +these same species may occasionally revert to some of the characters of +their ancient progenitors. Although new and important modifications may +not arise from reversion and analogous variation, such modifications +will add to the beautiful and harmonious diversity of nature. + + +Whatever the cause may be of each slight difference in the offspring +from their parents—and a cause for each must exist—it is the steady +accumulation, through natural selection, of such differences, when +beneficial to the individual, that gives rise to all the more important +modifications of structure, by which the innumerable beings on the face +of this earth are enabled to struggle with each other, and the best +adapted to survive. + + + + +CHAPTER VI. +DIFFICULTIES ON THEORY. + + +Difficulties on the theory of descent with modification. Transitions. +Absence or rarity of transitional varieties. Transitions in habits of +life. Diversified habits in the same species. Species with habits +widely different from those of their allies. Organs of extreme +perfection. Means of transition. Cases of difficulty. Natura non facit +saltum. Organs of small importance. Organs not in all cases absolutely +perfect. The law of Unity of Type and of the Conditions of Existence +embraced by the theory of Natural Selection. + + +Long before having arrived at this part of my work, a crowd of +difficulties will have occurred to the reader. Some of them are so +grave that to this day I can never reflect on them without being +staggered; but, to the best of my judgment, the greater number are only +apparent, and those that are real are not, I think, fatal to my theory. + +These difficulties and objections may be classed under the following +heads:— + +Firstly, why, if species have descended from other species by +insensibly fine gradations, do we not everywhere see innumerable +transitional forms? Why is not all nature in confusion instead of the +species being, as we see them, well defined? + +Secondly, is it possible that an animal having, for instance, the +structure and habits of a bat, could have been formed by the +modification of some animal with wholly different habits? Can we +believe that natural selection could produce, on the one hand, organs +of trifling importance, such as the tail of a giraffe, which serves as +a fly-flapper, and, on the other hand, organs of +such wonderful structure, as the eye, of which we hardly as yet fully +understand the inimitable perfection? + +Thirdly, can instincts be acquired and modified through natural +selection? What shall we say to so marvellous an instinct as that which +leads the bee to make cells, which have practically anticipated the +discoveries of profound mathematicians? + +Fourthly, how can we account for species, when crossed, being sterile +and producing sterile offspring, whereas, when varieties are crossed, +their fertility is unimpaired? + +The two first heads shall be here discussed—Instinct and Hybridism in +separate chapters. + +_On the absence or rarity of transitional varieties._—As natural +selection acts solely by the preservation of profitable modifications, +each new form will tend in a fully-stocked country to take the place +of, and finally to exterminate, its own less improved parent or other +less-favoured forms with which it comes into competition. Thus +extinction and natural selection will, as we have seen, go hand in +hand. Hence, if we look at each species as descended from some other +unknown form, both the parent and all the transitional varieties will +generally have been exterminated by the very process of formation and +perfection of the new form. + +But, as by this theory innumerable transitional forms must have +existed, why do we not find them embedded in countless numbers in the +crust of the earth? It will be much more convenient to discuss this +question in the chapter on the Imperfection of the geological record; +and I will here only state that I believe the answer mainly lies in the +record being incomparably less perfect than is generally supposed; the +imperfection of the record being chiefly due to organic beings not +inhabiting +profound depths of the sea, and to their remains being embedded and +preserved to a future age only in masses of sediment sufficiently thick +and extensive to withstand an enormous amount of future degradation; +and such fossiliferous masses can be accumulated only where much +sediment is deposited on the shallow bed of the sea, whilst it slowly +subsides. These contingencies will concur only rarely, and after +enormously long intervals. Whilst the bed of the sea is stationary or +is rising, or when very little sediment is being deposited, there will +be blanks in our geological history. The crust of the earth is a vast +museum; but the natural collections have been made only at intervals of +time immensely remote. + +But it may be urged that when several closely-allied species inhabit +the same territory we surely ought to find at the present time many +transitional forms. Let us take a simple case: in travelling from north +to south over a continent, we generally meet at successive intervals +with closely allied or representative species, evidently filling nearly +the same place in the natural economy of the land. These representative +species often meet and interlock; and as the one becomes rarer and +rarer, the other becomes more and more frequent, till the one replaces +the other. But if we compare these species where they intermingle, they +are generally as absolutely distinct from each other in every detail of +structure as are specimens taken from the metropolis inhabited by each. +By my theory these allied species have descended from a common parent; +and during the process of modification, each has become adapted to the +conditions of life of its own region, and has supplanted and +exterminated its original parent and all the transitional varieties +between its past and present states. Hence we ought not to expect at +the +present time to meet with numerous transitional varieties in each +region, though they must have existed there, and may be embedded there +in a fossil condition. But in the intermediate region, having +intermediate conditions of life, why do we not now find closely-linking +intermediate varieties? This difficulty for a long time quite +confounded me. But I think it can be in large part explained. + +In the first place we should be extremely cautious in inferring, +because an area is now continuous, that it has been continuous during a +long period. Geology would lead us to believe that almost every +continent has been broken up into islands even during the later +tertiary periods; and in such islands distinct species might have been +separately formed without the possibility of intermediate varieties +existing in the intermediate zones. By changes in the form of the land +and of climate, marine areas now continuous must often have existed +within recent times in a far less continuous and uniform condition than +at present. But I will pass over this way of escaping from the +difficulty; for I believe that many perfectly defined species have been +formed on strictly continuous areas; though I do not doubt that the +formerly broken condition of areas now continuous has played an +important part in the formation of new species, more especially with +freely-crossing and wandering animals. + +In looking at species as they are now distributed over a wide area, we +generally find them tolerably numerous over a large territory, then +becoming somewhat abruptly rarer and rarer on the confines, and finally +disappearing. Hence the neutral territory between two representative +species is generally narrow in comparison with the territory proper to +each. We see the same fact in ascending mountains, and sometimes +it is quite remarkable how abruptly, as Alph. De Candolle has observed, +a common alpine species disappears. The same fact has been noticed by +Forbes in sounding the depths of the sea with the dredge. To those who +look at climate and the physical conditions of life as the +all-important elements of distribution, these facts ought to cause +surprise, as climate and height or depth graduate away insensibly. But +when we bear in mind that almost every species, even in its metropolis, +would increase immensely in numbers, were it not for other competing +species; that nearly all either prey on or serve as prey for others; in +short, that each organic being is either directly or indirectly related +in the most important manner to other organic beings, we must see that +the range of the inhabitants of any country by no means exclusively +depends on insensibly changing physical conditions, but in large part +on the presence of other species, on which it depends, or by which it +is destroyed, or with which it comes into competition; and as these +species are already defined objects (however they may have become so), +not blending one into another by insensible gradations, the range of +any one species, depending as it does on the range of others, will tend +to be sharply defined. Moreover, each species on the confines of its +range, where it exists in lessened numbers, will, during fluctuations +in the number of its enemies or of its prey, or in the seasons, be +extremely liable to utter extermination; and thus its geographical +range will come to be still more sharply defined. + +If I am right in believing that allied or representative species, when +inhabiting a continuous area, are generally so distributed that each +has a wide range, with a comparatively narrow neutral territory between +them, in which they become rather suddenly rarer and rarer; then, as +varieties do not essentially differ from species, +the same rule will probably apply to both; and if we in imagination +adapt a varying species to a very large area, we shall have to adapt +two varieties to two large areas, and a third variety to a narrow +intermediate zone. The intermediate variety, consequently, will exist +in lesser numbers from inhabiting a narrow and lesser area; and +practically, as far as I can make out, this rule holds good with +varieties in a state of nature. I have met with striking instances of +the rule in the case of varieties intermediate between well-marked +varieties in the genus Balanus. And it would appear from information +given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that generally +when varieties intermediate between two other forms occur, they are +much rarer numerically than the forms which they connect. Now, if we +may trust these facts and inferences, and therefore conclude that +varieties linking two other varieties together have generally existed +in lesser numbers than the forms which they connect, then, I think, we +can understand why intermediate varieties should not endure for very +long periods;—why as a general rule they should be exterminated and +disappear, sooner than the forms which they originally linked together. + +For any form existing in lesser numbers would, as already remarked, run +a greater chance of being exterminated than one existing in large +numbers; and in this particular case the intermediate form would be +eminently liable to the inroads of closely allied forms existing on +both sides of it. But a far more important consideration, as I believe, +is that, during the process of further modification, by which two +varieties are supposed on my theory to be converted and perfected into +two distinct species, the two which exist in larger numbers from +inhabiting larger areas, will have a great advantage over the +intermediate variety, which exists +in smaller numbers in a narrow and intermediate zone. For forms +existing in larger numbers will always have a better chance, within any +given period, of presenting further favourable variations for natural +selection to seize on, than will the rarer forms which exist in lesser +numbers. Hence, the more common forms, in the race for life, will tend +to beat and supplant the less common forms, for these will be more +slowly modified and improved. It is the same principle which, as I +believe, accounts for the common species in each country, as shown in +the second chapter, presenting on an average a greater number of +well-marked varieties than do the rarer species. I may illustrate what +I mean by supposing three varieties of sheep to be kept, one adapted to +an extensive mountainous region; a second to a comparatively narrow, +hilly tract; and a third to wide plains at the base; and that the +inhabitants are all trying with equal steadiness and skill to improve +their stocks by selection; the chances in this case will be strongly in +favour of the great holders on the mountains or on the plains improving +their breeds more quickly than the small holders on the intermediate +narrow, hilly tract; and consequently the improved mountain or plain +breed will soon take the place of the less improved hill breed; and +thus the two breeds, which originally existed in greater numbers, will +come into close contact with each other, without the interposition of +the supplanted, intermediate hill-variety. + +To sum up, I believe that species come to be tolerably well-defined +objects, and do not at any one period present an inextricable chaos of +varying and intermediate links: firstly, because new varieties are very +slowly formed, for variation is a very slow process, and natural +selection can do nothing until favourable variations chance to occur, +and until a place in the natural polity +of the country can be better filled by some modification of some one or +more of its inhabitants. And such new places will depend on slow +changes of climate, or on the occasional immigration of new +inhabitants, and, probably, in a still more important degree, on some +of the old inhabitants becoming slowly modified, with the new forms +thus produced and the old ones acting and reacting on each other. So +that, in any one region and at any one time, we ought only to see a few +species presenting slight modifications of structure in some degree +permanent; and this assuredly we do see. + +Secondly, areas now continuous must often have existed within the +recent period in isolated portions, in which many forms, more +especially amongst the classes which unite for each birth and wander +much, may have separately been rendered sufficiently distinct to rank +as representative species. In this case, intermediate varieties between +the several representative species and their common parent, must +formerly have existed in each broken portion of the land, but these +links will have been supplanted and exterminated during the process of +natural selection, so that they will no longer exist in a living state. + +Thirdly, when two or more varieties have been formed in different +portions of a strictly continuous area, intermediate varieties will, it +is probable, at first have been formed in the intermediate zones, but +they will generally have had a short duration. For these intermediate +varieties will, from reasons already assigned (namely from what we know +of the actual distribution of closely allied or representative species, +and likewise of acknowledged varieties), exist in the intermediate +zones in lesser numbers than the varieties which they tend to connect. +From this cause alone the intermediate +varieties will be liable to accidental extermination; and during the +process of further modification through natural selection, they will +almost certainly be beaten and supplanted by the forms which they +connect; for these from existing in greater numbers will, in the +aggregate, present more variation, and thus be further improved through +natural selection and gain further advantages. + +Lastly, looking not to any one time, but to all time, if my theory be +true, numberless intermediate varieties, linking most closely all the +species of the same group together, must assuredly have existed; but +the very process of natural selection constantly tends, as has been so +often remarked, to exterminate the parent forms and the intermediate +links. Consequently evidence of their former existence could be found +only amongst fossil remains, which are preserved, as we shall in a +future chapter attempt to show, in an extremely imperfect and +intermittent record. + +_On the origin and transitions of organic beings with peculiar habits +and structure_.—It has been asked by the opponents of such views as I +hold, how, for instance, a land carnivorous animal could have been +converted into one with aquatic habits; for how could the animal in its +transitional state have subsisted? It would be easy to show that within +the same group carnivorous animals exist having every intermediate +grade between truly aquatic and strictly terrestrial habits; and as +each exists by a struggle for life, it is clear that each is well +adapted in its habits to its place in nature. Look at the Mustela vison +of North America, which has webbed feet and which resembles an otter in +its fur, short legs, and form of tail; during summer this animal dives +for and preys on fish, but during the long winter +it leaves the frozen waters, and preys like other polecats on mice and +land animals. If a different case had been taken, and it had been asked +how an insectivorous quadruped could possibly have been converted into +a flying bat, the question would have been far more difficult, and I +could have given no answer. Yet I think such difficulties have very +little weight. + +Here, as on other occasions, I lie under a heavy disadvantage, for out +of the many striking cases which I have collected, I can give only one +or two instances of transitional habits and structures in closely +allied species of the same genus; and of diversified habits, either +constant or occasional, in the same species. And it seems to me that +nothing less than a long list of such cases is sufficient to lessen the +difficulty in any particular case like that of the bat. + +Look at the family of squirrels; here we have the finest gradation from +animals with their tails only slightly flattened, and from others, as +Sir J. Richardson has remarked, with the posterior part of their bodies +rather wide and with the skin on their flanks rather full, to the +so-called flying squirrels; and flying squirrels have their limbs and +even the base of the tail united by a broad expanse of skin, which +serves as a parachute and allows them to glide through the air to an +astonishing distance from tree to tree. We cannot doubt that each +structure is of use to each kind of squirrel in its own country, by +enabling it to escape birds or beasts of prey, or to collect food more +quickly, or, as there is reason to believe, by lessening the danger +from occasional falls. But it does not follow from this fact that the +structure of each squirrel is the best that it is possible to conceive +under all natural conditions. Let the climate and vegetation change, +let other competing rodents or new beasts of prey immigrate, or old +ones +become modified, and all analogy would lead us to believe that some at +least of the squirrels would decrease in numbers or become +exterminated, unless they also became modified and improved in +structure in a corresponding manner. Therefore, I can see no +difficulty, more especially under changing conditions of life, in the +continued preservation of individuals with fuller and fuller +flank-membranes, each modification being useful, each being propagated, +until by the accumulated effects of this process of natural selection, +a perfect so-called flying squirrel was produced. + +Now look at the Galeopithecus or flying lemur, which formerly was +falsely ranked amongst bats. It has an extremely wide flank-membrane, +stretching from the corners of the jaw to the tail, and including the +limbs and the elongated fingers: the flank membrane is, also, furnished +with an extensor muscle. Although no graduated links of structure, +fitted for gliding through the air, now connect the Galeopithecus with +the other Lemuridæ, yet I can see no difficulty in supposing that such +links formerly existed, and that each had been formed by the same steps +as in the case of the less perfectly gliding squirrels; and that each +grade of structure had been useful to its possessor. Nor can I see any +insuperable difficulty in further believing it possible that the +membrane-connected fingers and fore-arm of the Galeopithecus might be +greatly lengthened by natural selection; and this, as far as the organs +of flight are concerned, would convert it into a bat. In bats which +have the wing-membrane extended from the top of the shoulder to the +tail, including the hind-legs, we perhaps see traces of an apparatus +originally constructed for gliding through the air rather than for +flight. + +If about a dozen genera of birds had become extinct or were unknown, +who would have ventured to have +surmised that birds might have existed which used their wings solely as +flappers, like the logger-headed duck (Micropterus of Eyton); as fins +in the water and front legs on the land, like the penguin; as sails, +like the ostrich; and functionally for no purpose, like the Apteryx. +Yet the structure of each of these birds is good for it, under the +conditions of life to which it is exposed, for each has to live by a +struggle; but it is not necessarily the best possible under all +possible conditions. It must not be inferred from these remarks that +any of the grades of wing-structure here alluded to, which perhaps may +all have resulted from disuse, indicate the natural steps by which +birds have acquired their perfect power of flight; but they serve, at +least, to show what diversified means of transition are possible. + +Seeing that a few members of such water-breathing classes as the +Crustacea and Mollusca are adapted to live on the land, and seeing that +we have flying birds and mammals, flying insects of the most +diversified types, and formerly had flying reptiles, it is conceivable +that flying-fish, which now glide far through the air, slightly rising +and turning by the aid of their fluttering fins, might have been +modified into perfectly winged animals. If this had been effected, who +would have ever imagined that in an early transitional state they had +been inhabitants of the open ocean, and had used their incipient organs +of flight exclusively, as far as we know, to escape being devoured by +other fish? + +When we see any structure highly perfected for any particular habit, as +the wings of a bird for flight, we should bear in mind that animals +displaying early transitional grades of the structure will seldom +continue to exist to the present day, for they will have been +supplanted by the very process of perfection through natural selection. +Furthermore, we may conclude that transitional +grades between structures fitted for very different habits of life will +rarely have been developed at an early period in great numbers and +under many subordinate forms. Thus, to return to our imaginary +illustration of the flying-fish, it does not seem probable that fishes +capable of true flight would have been developed under many subordinate +forms, for taking prey of many kinds in many ways, on the land and in +the water, until their organs of flight had come to a high stage of +perfection, so as to have given them a decided advantage over other +animals in the battle for life. Hence the chance of discovering species +with transitional grades of structure in a fossil condition will always +be less, from their having existed in lesser numbers, than in the case +of species with fully developed structures. + +I will now give two or three instances of diversified and of changed +habits in the individuals of the same species. When either case occurs, +it would be easy for natural selection to fit the animal, by some +modification of its structure, for its changed habits, or exclusively +for one of its several different habits. But it is difficult to tell, +and immaterial for us, whether habits generally change first and +structure afterwards; or whether slight modifications of structure lead +to changed habits; both probably often change almost simultaneously. Of +cases of changed habits it will suffice merely to allude to that of the +many British insects which now feed on exotic plants, or exclusively on +artificial substances. Of diversified habits innumerable instances +could be given: I have often watched a tyrant flycatcher (Saurophagus +sulphuratus) in South America, hovering over one spot and then +proceeding to another, like a kestrel, and at other times standing +stationary on the margin of water, and then dashing like a kingfisher +at a fish. In our own country the larger titmouse (Parus major) may be +seen climbing branches, almost like a creeper; it often, like a shrike, +kills small birds by blows on the head; and I have many times seen and +heard it hammering the seeds of the yew on a branch, and thus breaking +them like a nuthatch. In North America the black bear was seen by +Hearne swimming for hours with widely open mouth, thus catching, like a +whale, insects in the water. Even in so extreme a case as this, if the +supply of insects were constant, and if better adapted competitors did +not already exist in the country, I can see no difficulty in a race of +bears being rendered, by natural selection, more and more aquatic in +their structure and habits, with larger and larger mouths, till a +creature was produced as monstrous as a whale. + +As we sometimes see individuals of a species following habits widely +different from those both of their own species and of the other species +of the same genus, we might expect, on my theory, that such individuals +would occasionally have given rise to new species, having anomalous +habits, and with their structure either slightly or considerably +modified from that of their proper type. And such instances do occur in +nature. Can a more striking instance of adaptation be given than that +of a woodpecker for climbing trees and for seizing insects in the +chinks of the bark? Yet in North America there are woodpeckers which +feed largely on fruit, and others with elongated wings which chase +insects on the wing; and on the plains of La Plata, where not a tree +grows, there is a woodpecker, which in every essential part of its +organisation, even in its colouring, in the harsh tone of its voice, +and undulatory flight, told me plainly of its close blood-relationship +to our common species; yet it is a woodpecker which never climbs a +tree! + +Petrels are the most aërial and oceanic of birds, yet in the quiet +Sounds of Tierra del Fuego, the Puffinuria +berardi, in its general habits, in its astonishing power of diving, its +manner of swimming, and of flying when unwillingly it takes flight, +would be mistaken by any one for an auk or grebe; nevertheless, it is +essentially a petrel, but with many parts of its organisation +profoundly modified. On the other hand, the acutest observer by +examining the dead body of the water-ouzel would never have suspected +its sub-aquatic habits; yet this anomalous member of the strictly +terrestrial thrush family wholly subsists by diving,—grasping the +stones with its feet and using its wings under water. + +He who believes that each being has been created as we now see it, must +occasionally have felt surprise when he has met with an animal having +habits and structure not at all in agreement. What can be plainer than +that the webbed feet of ducks and geese are formed for swimming? yet +there are upland geese with webbed feet which rarely or never go near +the water; and no one except Audubon has seen the frigate-bird, which +has all its four toes webbed, alight on the surface of the sea. On the +other hand, grebes and coots are eminently aquatic, although their toes +are only bordered by membrane. What seems plainer than that the long +toes of grallatores are formed for walking over swamps and floating +plants, yet the water-hen is nearly as aquatic as the coot; and the +landrail nearly as terrestrial as the quail or partridge. In such +cases, and many others could be given, habits have changed without a +corresponding change of structure. The webbed feet of the upland goose +may be said to have become rudimentary in function, though not in +structure. In the frigate-bird, the deeply-scooped membrane between the +toes shows that structure has begun to change. + +He who believes in separate and innumerable acts of creation will say, +that in these cases it has pleased the +Creator to cause a being of one type to take the place of one of +another type; but this seems to me only restating the fact in dignified +language. He who believes in the struggle for existence and in the +principle of natural selection, will acknowledge that every organic +being is constantly endeavouring to increase in numbers; and that if +any one being vary ever so little, either in habits or structure, and +thus gain an advantage over some other inhabitant of the country, it +will seize on the place of that inhabitant, however different it may be +from its own place. Hence it will cause him no surprise that there +should be geese and frigate-birds with webbed feet, either living on +the dry land or most rarely alighting on the water; that there should +be long-toed corncrakes living in meadows instead of in swamps; that +there should be woodpeckers where not a tree grows; that there should +be diving thrushes, and petrels with the habits of auks. + +_Organs of extreme perfection and complication_.—To suppose that the +eye, with all its inimitable contrivances for adjusting the focus to +different distances, for admitting different amounts of light, and for +the correction of spherical and chromatic aberration, could have been +formed by natural selection, seems, I freely confess, absurd in the +highest possible degree. Yet reason tells me, that if numerous +gradations from a perfect and complex eye to one very imperfect and +simple, each grade being useful to its possessor, can be shown to +exist; if further, the eye does vary ever so slightly, and the +variations be inherited, which is certainly the case; and if any +variation or modification in the organ be ever useful to an animal +under changing conditions of life, then the difficulty of believing +that a perfect and complex eye could be formed by natural +selection, though insuperable by our imagination, can hardly be +considered real. How a nerve comes to be sensitive to light, hardly +concerns us more than how life itself first originated; but I may +remark that several facts make me suspect that any sensitive nerve may +be rendered sensitive to light, and likewise to those coarser +vibrations of the air which produce sound. + +In looking for the gradations by which an organ in any species has been +perfected, we ought to look exclusively to its lineal ancestors; but +this is scarcely ever possible, and we are forced in each case to look +to species of the same group, that is to the collateral descendants +from the same original parent-form, in order to see what gradations are +possible, and for the chance of some gradations having been transmitted +from the earlier stages of descent, in an unaltered or little altered +condition. Amongst existing Vertebrata, we find but a small amount of +gradation in the structure of the eye, and from fossil species we can +learn nothing on this head. In this great class we should probably have +to descend far beneath the lowest known fossiliferous stratum to +discover the earlier stages, by which the eye has been perfected. + +In the Articulata we can commence a series with an optic nerve merely +coated with pigment, and without any other mechanism; and from this low +stage, numerous gradations of structure, branching off in two +fundamentally different lines, can be shown to exist, until we reach a +moderately high stage of perfection. In certain crustaceans, for +instance, there is a double cornea, the inner one divided into facets, +within each of which there is a lens-shaped swelling. In other +crustaceans the transparent cones which are coated by pigment, and +which properly act only by excluding lateral pencils of light, are +convex at their upper ends +and must act by convergence; and at their lower ends there seems to be +an imperfect vitreous substance. With these facts, here far too briefly +and imperfectly given, which show that there is much graduated +diversity in the eyes of living crustaceans, and bearing in mind how +small the number of living animals is in proportion to those which have +become extinct, I can see no very great difficulty (not more than in +the case of many other structures) in believing that natural selection +has converted the simple apparatus of an optic nerve merely coated with +pigment and invested by transparent membrane, into an optical +instrument as perfect as is possessed by any member of the great +Articulate class. + +He who will go thus far, if he find on finishing this treatise that +large bodies of facts, otherwise inexplicable, can be explained by the +theory of descent, ought not to hesitate to go further, and to admit +that a structure even as perfect as the eye of an eagle might be formed +by natural selection, although in this case he does not know any of the +transitional grades. His reason ought to conquer his imagination; +though I have felt the difficulty far too keenly to be surprised at any +degree of hesitation in extending the principle of natural selection to +such startling lengths. + +It is scarcely possible to avoid comparing the eye to a telescope. We +know that this instrument has been perfected by the long-continued +efforts of the highest human intellects; and we naturally infer that +the eye has been formed by a somewhat analogous process. But may not +this inference be presumptuous? Have we any right to assume that the +Creator works by intellectual powers like those of man? If we must +compare the eye to an optical instrument, we ought in imagination to +take a thick layer of transparent tissue, with a nerve sensitive to +light beneath, and then suppose every +part of this layer to be continually changing slowly in density, so as +to separate into layers of different densities and thicknesses, placed +at different distances from each other, and with the surfaces of each +layer slowly changing in form. Further we must suppose that there is a +power always intently watching each slight accidental alteration in the +transparent layers; and carefully selecting each alteration which, +under varied circumstances, may in any way, or in any degree, tend to +produce a distincter image. We must suppose each new state of the +instrument to be multiplied by the million; and each to be preserved +till a better be produced, and then the old ones to be destroyed. In +living bodies, variation will cause the slight alterations, generation +will multiply them almost infinitely, and natural selection will pick +out with unerring skill each improvement. Let this process go on for +millions on millions of years; and during each year on millions of +individuals of many kinds; and may we not believe that a living optical +instrument might thus be formed as superior to one of glass, as the +works of the Creator are to those of man? + +If it could be demonstrated that any complex organ existed, which could +not possibly have been formed by numerous, successive, slight +modifications, my theory would absolutely break down. But I can find +out no such case. No doubt many organs exist of which we do not know +the transitional grades, more especially if we look to much-isolated +species, round which, according to my theory, there has been much +extinction. Or again, if we look to an organ common to all the members +of a large class, for in this latter case the organ must have been +first formed at an extremely remote period, since which all the many +members of the class have been developed; and in order to discover the +early transitional grades through which the organ has +passed, we should have to look to very ancient ancestral forms, long +since become extinct. + +We should be extremely cautious in concluding that an organ could not +have been formed by transitional gradations of some kind. Numerous +cases could be given amongst the lower animals of the same organ +performing at the same time wholly distinct functions; thus the +alimentary canal respires, digests, and excretes in the larva of the +dragon-fly and in the fish Cobites. In the Hydra, the animal may be +turned inside out, and the exterior surface will then digest and the +stomach respire. In such cases natural selection might easily +specialise, if any advantage were thus gained, a part or organ, which +had performed two functions, for one function alone, and thus wholly +change its nature by insensible steps. Two distinct organs sometimes +perform simultaneously the same function in the same individual; to +give one instance, there are fish with gills or branchiæ that breathe +the air dissolved in the water, at the same time that they breathe free +air in their swimbladders, this latter organ having a ductus +pneumaticus for its supply, and being divided by highly vascular +partitions. In these cases, one of the two organs might with ease be +modified and perfected so as to perform all the work by itself, being +aided during the process of modification by the other organ; and then +this other organ might be modified for some other and quite distinct +purpose, or be quite obliterated. + +The illustration of the swimbladder in fishes is a good one, because it +shows us clearly the highly important fact that an organ originally +constructed for one purpose, namely flotation, may be converted into +one for a wholly different purpose, namely respiration. The swimbladder +has, also, been worked in as an accessory to the auditory organs of +certain fish, or, for I do not know which +view is now generally held, a part of the auditory apparatus has been +worked in as a complement to the swimbladder. All physiologists admit +that the swimbladder is homologous, or “ideally similar,” in position +and structure with the lungs of the higher vertebrate animals: hence +there seems to me to be no great difficulty in believing that natural +selection has actually converted a swimbladder into a lung, or organ +used exclusively for respiration. + +I can, indeed, hardly doubt that all vertebrate animals having true +lungs have descended by ordinary generation from an ancient prototype, +of which we know nothing, furnished with a floating apparatus or +swimbladder. We can thus, as I infer from Professor Owen’s interesting +description of these parts, understand the strange fact that every +particle of food and drink which we swallow has to pass over the +orifice of the trachea, with some risk of falling into the lungs, +notwithstanding the beautiful contrivance by which the glottis is +closed. In the higher Vertebrata the branchiæ have wholly +disappeared—the slits on the sides of the neck and the loop-like course +of the arteries still marking in the embryo their former position. But +it is conceivable that the now utterly lost branchiæ might have been +gradually worked in by natural selection for some quite distinct +purpose: in the same manner as, on the view entertained by some +naturalists that the branchiæ and dorsal scales of Annelids are +homologous with the wings and wing-covers of insects, it is probable +that organs which at a very ancient period served for respiration have +been actually converted into organs of flight. + +In considering transitions of organs, it is so important to bear in +mind the probability of conversion from one function to another, that I +will give one more instance. Pedunculated cirripedes have two minute +folds of skin, +called by me the ovigerous frena, which serve, through the means of a +sticky secretion, to retain the eggs until they are hatched within the +sack. These cirripedes have no branchiæ, the whole surface of the body +and sack, including the small frena, serving for respiration. The +Balanidæ or sessile cirripedes, on the other hand, have no ovigerous +frena, the eggs lying loose at the bottom of the sack, in the +well-enclosed shell; but they have large folded branchiæ. Now I think +no one will dispute that the ovigerous frena in the one family are +strictly homologous with the branchiæ of the other family; indeed, they +graduate into each other. Therefore I do not doubt that little folds of +skin, which originally served as ovigerous frena, but which, likewise, +very slightly aided the act of respiration, have been gradually +converted by natural selection into branchiæ, simply through an +increase in their size and the obliteration of their adhesive glands. +If all pedunculated cirripedes had become extinct, and they have +already suffered far more extinction than have sessile cirripedes, who +would ever have imagined that the branchiæ in this latter family had +originally existed as organs for preventing the ova from being washed +out of the sack? + +Although we must be extremely cautious in concluding that any organ +could not possibly have been produced by successive transitional +gradations, yet, undoubtedly, grave cases of difficulty occur, some of +which will be discussed in my future work. + +One of the gravest is that of neuter insects, which are often very +differently constructed from either the males or fertile females; but +this case will be treated of in the next chapter. The electric organs +of fishes offer another case of special difficulty; it is impossible to +conceive by what steps these wondrous organs have been produced; but, +as Owen and others have remarked, +their intimate structure closely resembles that of common muscle; and +as it has lately been shown that Rays have an organ closely analogous +to the electric apparatus, and yet do not, as Matteuchi asserts, +discharge any electricity, we must own that we are far too ignorant to +argue that no transition of any kind is possible. + +The electric organs offer another and even more serious difficulty; for +they occur in only about a dozen fishes, of which several are widely +remote in their affinities. Generally when the same organ appears in +several members of the same class, especially if in members having very +different habits of life, we may attribute its presence to inheritance +from a common ancestor; and its absence in some of the members to its +loss through disuse or natural selection. But if the electric organs +had been inherited from one ancient progenitor thus provided, we might +have expected that all electric fishes would have been specially +related to each other. Nor does geology at all lead to the belief that +formerly most fishes had electric organs, which most of their modified +descendants have lost. The presence of luminous organs in a few +insects, belonging to different families and orders, offers a parallel +case of difficulty. Other cases could be given; for instance in plants, +the very curious contrivance of a mass of pollen-grains, borne on a +foot-stalk with a sticky gland at the end, is the same in Orchis and +Asclepias,—genera almost as remote as possible amongst flowering +plants. In all these cases of two very distinct species furnished with +apparently the same anomalous organ, it should be observed that, +although the general appearance and function of the organ may be the +same, yet some fundamental difference can generally be detected. I am +inclined to believe that in nearly the same way as two men have +sometimes independently hit on +the very same invention, so natural selection, working for the good of +each being and taking advantage of analogous variations, has sometimes +modified in very nearly the same manner two parts in two organic +beings, which owe but little of their structure in common to +inheritance from the same ancestor. + +Although in many cases it is most difficult to conjecture by what +transitions an organ could have arrived at its present state; yet, +considering that the proportion of living and known forms to the +extinct and unknown is very small, I have been astonished how rarely an +organ can be named, towards which no transitional grade is known to +lead. The truth of this remark is indeed shown by that old canon in +natural history of “Natura non facit saltum.” We meet with this +admission in the writings of almost every experienced naturalist; or, +as Milne Edwards has well expressed it, nature is prodigal in variety, +but niggard in innovation. Why, on the theory of Creation, should this +be so? Why should all the parts and organs of many independent beings, +each supposed to have been separately created for its proper place in +nature, be so invariably linked together by graduated steps? Why should +not Nature have taken a leap from structure to structure? On the theory +of natural selection, we can clearly understand why she should not; for +natural selection can act only by taking advantage of slight successive +variations; she can never take a leap, but must advance by the shortest +and slowest steps. + +_Organs of little apparent importance_.—As natural selection acts by +life and death,—by the preservation of individuals with any favourable +variation, and by the destruction of those with any unfavourable +deviation of structure,—I have sometimes felt much difficulty in +understanding the origin of simple parts, of which the importance does +not seem sufficient to cause the preservation of successively varying +individuals. I have sometimes felt as much difficulty, though of a very +different kind, on this head, as in the case of an organ as perfect and +complex as the eye. + +In the first place, we are much too ignorant in regard to the whole +economy of any one organic being, to say what slight modifications +would be of importance or not. In a former chapter I have given +instances of most trifling characters, such as the down on fruit and +the colour of the flesh, which, from determining the attacks of insects +or from being correlated with constitutional differences, might +assuredly be acted on by natural selection. The tail of the giraffe +looks like an artificially constructed fly-flapper; and it seems at +first incredible that this could have been adapted for its present +purpose by successive slight modifications, each better and better, for +so trifling an object as driving away flies; yet we should pause before +being too positive even in this case, for we know that the distribution +and existence of cattle and other animals in South America absolutely +depends on their power of resisting the attacks of insects: so that +individuals which could by any means defend themselves from these small +enemies, would be able to range into new pastures and thus gain a great +advantage. It is not that the larger quadrupeds are actually destroyed +(except in some rare cases) by the 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 in nearly the same state, although +now become of very slight use; and any actually injurious deviations in +their structure will always have been checked by natural selection. +Seeing how important an organ of locomotion the tail is in most aquatic +animals, its general presence and use for many purposes in so many land +animals, which in their lungs or modified swim-bladders betray their +aquatic origin, may perhaps be thus accounted for. A well-developed +tail having been formed in an aquatic animal, it might subsequently +come to be worked in for all sorts of purposes, as a fly-flapper, an +organ of prehension, or as an aid in turning, as with the dog, though +the aid must be slight, for the hare, with hardly any tail, can double +quickly enough. + +In the second place, we may sometimes attribute importance to +characters which are really of very little importance, and which have +originated from quite secondary causes, independently of natural +selection. We should remember that climate, food, etc., probably have +some little direct influence on the organisation; that characters +reappear from the law of reversion; that correlation of growth will +have had a most important influence in modifying various structures; +and finally, that sexual selection will often have largely modified the +external characters of animals having a will, to give one male an +advantage in fighting with another or in charming the females. Moreover +when a modification of structure has primarily arisen from the above or +other unknown causes, it may at first have been of no advantage to the +species, but may subsequently have been taken advantage of by the +descendants of the species under new conditions of life and with newly +acquired habits. + +To give a few instances to illustrate these latter +remarks. If green woodpeckers alone had existed, and we did not know +that there were many black and pied kinds, I dare say that we should +have thought that the green colour was a beautiful adaptation to hide +this tree-frequenting bird from its enemies; and consequently that it +was a character of importance and might have been acquired through +natural selection; as it is, I have no doubt that the colour is due to +some quite distinct cause, probably to sexual selection. A trailing +bamboo in the Malay Archipelago climbs the loftiest trees by the aid of +exquisitely constructed hooks clustered around the ends of the +branches, and this contrivance, no doubt, is of the highest service to +the plant; but as we see nearly similar hooks on many trees which are +not climbers, the hooks on the bamboo may have arisen from unknown laws +of growth, and have been subsequently taken advantage of by the plant +undergoing further modification and becoming a climber. The naked skin +on the head of a vulture is generally looked at as a direct adaptation +for wallowing in putridity; and so it may be, or it may possibly be due +to the direct action of putrid matter; but we should be very cautious +in drawing any such inference, when we see that the skin on the head of +the clean-feeding male turkey is likewise naked. The sutures in the +skulls of young mammals have been advanced as a beautiful adaptation +for aiding parturition, and no doubt they facilitate, or may be +indispensable for this act; but as sutures occur in the skulls of young +birds and reptiles, which have only to escape from a broken egg, we may +infer that this structure has arisen from the laws of growth, and has +been taken advantage of in the parturition of the higher animals. + +We are profoundly ignorant of the causes producing slight and +unimportant variations; and we are immediately +made conscious of this by reflecting on the differences in the breeds +of our domesticated animals in different countries,—more especially in +the less civilized countries where there has been but little artificial +selection. Careful observers are convinced that a damp climate affects +the growth of the hair, and that with the hair the horns are +correlated. Mountain breeds always differ from lowland breeds; and a +mountainous country would probably affect the hind limbs from +exercising them more, and possibly even the form of the pelvis; and +then by the law of homologous variation, the front limbs and even the +head would probably be affected. The shape, also, of the pelvis might +affect by pressure the shape of the head of the young in the womb. The +laborious breathing necessary in high regions would, we have some +reason to believe, increase the size of the chest; and again +correlation would come into play. Animals kept by savages in different +countries often have to struggle for their own subsistence, and would +be exposed to a certain extent to natural selection, and individuals +with slightly different constitutions would succeed best under +different climates; and there is reason to believe that constitution +and colour are correlated. A good observer, also, states that in cattle +susceptibility to the attacks of flies is correlated with colour, as is +the liability to be poisoned by certain plants; so that colour would be +thus subjected to the action of natural selection. But we are far too +ignorant to speculate on the relative importance of the several known +and unknown laws of variation; and I have here alluded to them only to +show that, if we are unable to account for the characteristic +differences of our domestic breeds, which nevertheless we generally +admit to have arisen through ordinary generation, we ought not to lay +too much stress on our +ignorance of the precise cause of the slight analogous differences +between species. I might have adduced for this same purpose the +differences between the races of man, which are so strongly marked; I +may add that some little light can apparently be thrown on the origin +of these differences, chiefly through sexual selection of a particular +kind, but without here entering on copious details my reasoning would +appear frivolous. + +The foregoing remarks lead me to say a few words on the protest lately +made by some naturalists, against the utilitarian doctrine that every +detail of structure has been produced for the good of its possessor. +They believe that very many structures have been created for beauty in +the eyes of man, or for mere variety. This doctrine, if true, would be +absolutely fatal to my theory. Yet I fully admit that many structures +are of no direct use to their possessors. Physical conditions probably +have had some little effect on structure, quite independently of any +good thus gained. Correlation of growth has no doubt played a most +important part, and a useful modification of one part will often have +entailed on other parts diversified changes of no direct use. So again +characters which formerly were useful, or which formerly had arisen +from correlation of growth, or from other unknown cause, may reappear +from the law of reversion, though now of no direct use. The effects of +sexual selection, when displayed in beauty to charm the females, can be +called useful only in rather a forced sense. But by far the most +important consideration is that the chief part of the organisation of +every being is simply due to inheritance; and consequently, though each +being assuredly is well fitted for its place in nature, many structures +now have no direct relation to the habits of life of each species. +Thus, we can hardly believe that the webbed feet of the upland +goose or of the frigate-bird are of special use to these birds; we +cannot believe that the same bones in the arm of the monkey, in the +fore leg of the horse, in the wing of the bat, and in the flipper of +the seal, are of special use to these animals. We may safely attribute +these structures to inheritance. But to the progenitor of the upland +goose and of the frigate-bird, webbed feet no doubt were as useful as +they now are to the most aquatic of existing birds. So we may believe +that the progenitor of the seal had not a 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, which have been inherited from a common progenitor, were formerly +of more special use to that progenitor, or its progenitors, than they +now are to these animals having such widely diversified habits. +Therefore we may infer that these several bones might have been +acquired through natural selection, subjected formerly, as now, to the +several laws of inheritance, reversion, correlation of growth, etc. +Hence every detail of structure in every living creature (making some +little allowance for the direct action of physical conditions) may be +viewed, either as having been of special use to some ancestral form, or +as being now of special use to the descendants of this form—either +directly, or indirectly through the complex laws of growth. + +Natural selection cannot possibly produce any modification in any one +species exclusively for the good of another species; though throughout +nature one species incessantly takes advantage of, and profits by, the +structure of another. But natural selection can and does often produce +structures for the direct injury of other species, as we see in the +fang of the adder, and in the ovipositor of the ichneumon, by which its +eggs are deposited +in the living bodies of other insects. If it could be proved that any +part of the structure of any one species had been formed for the +exclusive good of another species, it would annihilate my theory, for +such could not have been produced through natural selection. Although +many statements may be found in works on natural history to this +effect, I cannot find even one which seems to me of any weight. It is +admitted that the rattlesnake has a poison-fang for its own defence and +for the destruction of its prey; but some authors suppose that at the +same time this snake is furnished with a rattle for its own injury, +namely, to warn its prey to escape. I would almost as soon believe that +the cat curls the end of its tail when preparing to spring, in order to +warn the doomed mouse. But I have not space here to enter on this and +other such cases. + +Natural selection will never produce in a being anything injurious to +itself, for natural selection acts solely by and for the good of each. +No organ will be formed, as Paley has remarked, for the purpose of +causing pain or for doing an injury to its possessor. If a fair balance +be struck between the good and evil caused by each part, each will be +found on the whole advantageous. After the lapse of time, under +changing conditions of life, if any part comes to be injurious, it will +be modified; or if it be not so, the being will become extinct, as +myriads have become extinct. + +Natural selection tends only to make each organic being as perfect as, +or slightly more perfect than, the other inhabitants of the same +country with which it has to struggle for existence. And we see that +this is the degree of perfection attained under nature. The endemic +productions of New Zealand, for instance, are perfect one compared with +another; but they are now rapidly yielding before the advancing legions +of plants +and animals introduced from Europe. Natural selection will not produce +absolute perfection, nor do we always meet, as far as we can judge, +with this high standard under nature. The correction for the aberration +of light is said, on high authority, not to be perfect even in that +most perfect organ, the eye. If our reason leads us to admire with +enthusiasm a multitude of inimitable contrivances in nature, this same +reason tells us, though we may easily err on both sides, that some +other contrivances are less perfect. Can we consider the sting of the +wasp or of the bee as perfect, which, when used against many attacking +animals, cannot be withdrawn, owing to the backward serratures, and so +inevitably causes the death of the insect by tearing out its viscera? + +If we look at the sting of the bee, as having originally existed in a +remote progenitor as a boring and serrated instrument, like that in so +many members of the same great order, and which has been modified but +not perfected for its present purpose, with the poison originally +adapted to cause galls subsequently intensified, we can perhaps +understand how it is that the use of the sting should so often cause +the insect’s own death: for if on the whole the power of stinging be +useful to the community, it will fulfil all the requirements of natural +selection, though it may cause the death of some few members. If we +admire the truly wonderful power of scent by which the males of many +insects find their females, can we admire the production for this +single purpose of thousands of drones, which are utterly useless to the +community for any other end, and which are ultimately slaughtered by +their industrious and sterile sisters? It may be difficult, but we +ought to admire the savage instinctive hatred of the queen-bee, which +urges her instantly to destroy the +young queens her daughters as soon as born, or to perish herself in the +combat; for undoubtedly this is for the good of the community; and +maternal love or maternal hatred, though the latter fortunately is most +rare, is all the same to the inexorable principle of natural selection. +If we admire the several ingenious contrivances, by which the flowers +of the orchis and of many other plants are fertilised through insect +agency, can we consider as equally perfect the elaboration by our +fir-trees of dense clouds of pollen, in order that a few granules may +be wafted by a chance breeze on to the ovules? + +_Summary of Chapter_.—We have in this chapter discussed some of the +difficulties and objections which may be urged against my theory. Many +of them are very grave; but I think that in the discussion light has +been thrown on several facts, which on the theory of independent acts +of creation are utterly obscure. We have seen that species at any one +period are not indefinitely variable, and are not linked together by a +multitude of intermediate gradations, partly because the process of +natural selection will always be very slow, and will act, at any one +time, only on a very few forms; and partly because the very process of +natural selection almost implies the continual supplanting and +extinction of preceding and intermediate gradations. Closely allied +species, now living on a continuous area, must often have been formed +when the area was not continuous, and when the conditions of life did +not insensibly graduate away from one part to another. When two +varieties are formed in two districts of a continuous area, an +intermediate variety will often be formed, fitted for an intermediate +zone; but from reasons assigned, the intermediate variety will usually +exist in lesser numbers than +the two forms which it connects; consequently the two latter, during +the course of further modification, from existing in greater numbers, +will have a great advantage over the less numerous intermediate +variety, and will thus generally succeed in supplanting and +exterminating it. + +We have seen in this chapter how cautious we should be in concluding +that the most different habits of life could not graduate into each +other; that a bat, for instance, could not have been formed by natural +selection from an animal which at first could only glide through the +air. + +We have seen that a species may under new conditions of life change its +habits, or have diversified habits, with some habits very unlike those +of its nearest congeners. Hence we can understand, bearing in mind that +each organic being is trying to live wherever it can live, how it has +arisen that there are upland geese with webbed feet, ground +woodpeckers, diving thrushes, and petrels with the habits of auks. + +Although the belief that an organ so perfect as the eye could have been +formed by natural selection, is more than enough to stagger any one; +yet in the case of any organ, if we know of a long series of gradations +in complexity, each good for its possessor, then, under changing +conditions of life, there is no logical impossibility in the +acquirement of any conceivable degree of perfection through natural +selection. In the cases in which we know of no intermediate or +transitional states, we should be very cautious in concluding that none +could have existed, for the homologies of many organs and their +intermediate states show that wonderful metamorphoses in function are +at least possible. For instance, a swim-bladder has apparently been +converted into an air-breathing lung. The same organ having performed +simultaneously very different functions, and then having been +specialised for one function; and two very distinct organs having +performed at the same time the same function, the one having been +perfected whilst aided by the other, must often have largely +facilitated transitions. + +We are far too ignorant, in almost every case, to be enabled to assert +that any part or organ is so unimportant for the welfare of a species, +that modifications in its structure could not have been slowly +accumulated by means of natural selection. But we may confidently +believe that many modifications, wholly due to the laws of growth, and +at first in no way advantageous to a species, have been subsequently +taken advantage of by the still further modified descendants of this +species. We may, also, believe that a part formerly of high importance +has often been retained (as the tail of an aquatic animal by its +terrestrial descendants), though it has become of such small importance +that it could not, in its present state, have been acquired by natural +selection,—a power which acts solely by the preservation of profitable +variations in the struggle for life. + +Natural selection will produce nothing in one species for the exclusive +good or injury of another; though it may well produce parts, organs, +and excretions highly useful or even indispensable, or highly injurious +to another species, but in all cases at the same time useful to the +owner. Natural selection in each well-stocked country, must act chiefly +through the competition of the inhabitants one with another, and +consequently will produce perfection, or strength in the battle for +life, only according to the standard of that country. Hence the +inhabitants of one country, generally the smaller one, will often +yield, as we see they do yield, to the inhabitants of another and +generally larger country. For in +the larger country there will have existed more individuals, and more +diversified forms, and the competition will have been severer, and thus +the standard of perfection will have been rendered higher. Natural +selection will not necessarily produce absolute perfection; nor, as far +as we can judge by our limited faculties, can absolute perfection be +everywhere found. + +On the theory of natural selection we can clearly understand the full +meaning of that old canon in natural history, “Natura non facit +saltum.” This canon, if we look only to the present inhabitants of the +world, is not strictly correct, but if we include all those of past +times, it must by my theory be strictly true. + +It is generally acknowledged that all organic beings have been formed +on two great laws—Unity of Type, and the Conditions of Existence. By +unity of type is meant that fundamental agreement in structure, which +we see in organic beings of the same class, and which is quite +independent of their habits of life. On my theory, unity of type is +explained by unity of descent. The expression of conditions of +existence, so often insisted on by the illustrious Cuvier, is fully +embraced by the principle of natural selection. For natural selection +acts by either now adapting the varying parts of each being to its +organic and inorganic conditions of life; or by having adapted them +during long-past periods of time: the adaptations being aided in some +cases by use and disuse, being slightly affected by the direct action +of the external conditions of life, and being in all cases subjected to +the several laws of growth. Hence, in fact, the law of the Conditions +of Existence is the higher law; as it includes, through the inheritance +of former adaptations, that of Unity of Type. + + + + +CHAPTER VII. +INSTINCT. + + +Instincts comparable with habits, but different in their origin. +Instincts graduated. Aphides and ants. Instincts variable. Domestic +instincts, their origin. Natural instincts of the cuckoo, ostrich, and +parasitic bees. Slave-making ants. Hive-bee, its cell-making instinct. +Difficulties on the theory of the Natural Selection of instincts. +Neuter or sterile insects. Summary. + + +The subject of instinct might have been worked into the previous +chapters; but I have thought that it would be more convenient to treat +the subject separately, especially as so wonderful an instinct as that +of the hive-bee making its cells will probably have occurred to many +readers, as a difficulty sufficient to overthrow my whole theory. I +must premise, that I have nothing to do with the origin of the primary +mental powers, any more than I have with that of life itself. We are +concerned only with the diversities of instinct and of the other mental +qualities of animals within the same class. + +I will not attempt any definition of instinct. It would be easy to show +that several distinct mental actions are commonly embraced by this +term; but every one understands what is meant, when it is said that +instinct impels the cuckoo to migrate and to lay her eggs in other +birds’ nests. An action, which we ourselves should require experience +to enable us to perform, when performed by an animal, more especially +by a very young one, without any experience, and when performed by many +individuals in the same way, without their knowing for what purpose it +is performed, is usually said to be instinctive. +But I could show that none of these characters of instinct are +universal. A little dose, as Pierre Huber expresses it, of judgment or +reason, often comes into play, even in animals very low in the scale of +nature. + +Frederick Cuvier and several of the older metaphysicians have compared +instinct with habit. This comparison gives, I think, a remarkably +accurate notion of the frame of mind under which an instinctive action +is performed, but not of its origin. How unconsciously many habitual +actions are performed, indeed not rarely in direct opposition to our +conscious will! yet they may be modified by the will or reason. Habits +easily become associated with other habits, and with certain periods of +time and states of the body. When once acquired, they often remain +constant throughout life. Several other points of resemblance between +instincts and habits could be pointed out. As in repeating a well-known +song, so in instincts, one action follows another by a sort of rhythm; +if a person be interrupted in a song, or in repeating anything by rote, +he is generally forced to go back to recover the habitual train of +thought: so P. Huber found it was with a caterpillar, which makes a +very complicated hammock; for if he took a caterpillar which had +completed its hammock up to, say, the sixth stage of construction, and +put it into a hammock completed up only to the third stage, the +caterpillar simply re-performed the fourth, fifth, and sixth stages of +construction. If, however, a caterpillar were taken out of a hammock +made up, for instance, to the third stage, and were put into one +finished up to the sixth stage, so that much of its work was already +done for it, far from feeling the benefit of this, it was much +embarrassed, and, in order to complete its hammock, seemed forced to +start from the third stage, where it had left off, and thus tried to +complete the already finished work. + + +If we suppose any habitual action to become inherited—and I think it +can be shown that this does sometimes happen—then the resemblance +between what originally was a habit and an instinct becomes so close as +not to be distinguished. If Mozart, instead of playing the pianoforte +at three years old with wonderfully little practice, had played a tune +with no practice at all, he might truly be said to have done so +instinctively. But it would be the most serious error to suppose that +the greater number of instincts have been acquired by habit in one +generation, and then transmitted by inheritance to succeeding +generations. It can be clearly shown that the most wonderful instincts +with which we are acquainted, namely, those of the hive-bee and of many +ants, could not possibly have been thus acquired. + +It will be universally admitted that instincts are as important as +corporeal structure for the welfare of each species, under its present +conditions of life. Under changed conditions of life, it is at least +possible that slight modifications of instinct might be profitable to a +species; and if it can be shown that instincts do vary ever so little, +then I can see no difficulty in natural selection preserving and +continually accumulating variations of instinct to any extent that may +be profitable. It is thus, as I believe, that all the most complex and +wonderful instincts have originated. As modifications of corporeal +structure arise from, and are increased by, use or habit, and are +diminished or lost by disuse, so I do not doubt it has been with +instincts. But I believe that the effects of habit are of quite +subordinate importance to the effects of the natural selection of what +may be called accidental variations of instincts;—that is of variations +produced by the same unknown causes which produce slight deviations of +bodily structure. + +No complex instinct can possibly be produced through +natural selection, except by the slow and gradual accumulation of +numerous, slight, yet profitable, variations. Hence, as in the case of +corporeal structures, we ought to find in nature, not the actual +transitional gradations by which each complex instinct has been +acquired—for these could be found only in the lineal ancestors of each +species—but we ought to find in the collateral lines of descent some +evidence of such gradations; or we ought at least to be able to show +that gradations of some kind are possible; and this we certainly can +do. I have been surprised to find, making allowance for the instincts +of animals having been but little observed except in Europe and North +America, and for no instinct being known amongst extinct species, how +very generally gradations, leading to the most complex instincts, can +be discovered. The canon of “Natura non facit saltum” applies with +almost equal force to instincts as to bodily organs. Changes of +instinct may sometimes be facilitated by the same species having +different instincts at different periods of life, or at different +seasons of the year, or when placed under different circumstances, +etc.; in which case either one or the other instinct might be preserved +by natural selection. And such instances of diversity of instinct in +the same species can be shown to occur in nature. + +Again as in the case of corporeal structure, and conformably with my +theory, the instinct of each species is good for itself, but has never, +as far as we can judge, been produced for the exclusive good of others. +One of the strongest instances of an animal apparently performing an +action for the sole good of another, with which I am acquainted, is +that of aphides voluntarily yielding their sweet excretion to ants: +that they do so voluntarily, the following facts show. I removed all +the ants from a group of about a dozen aphides on a dock-plant, +and prevented their attendance during several hours. After this +interval, I felt sure that the aphides would want to excrete. I watched +them for some time through a lens, but not one excreted; I then tickled +and stroked them with a hair in the same manner, as well as I could, as +the ants do with their antennæ; but not one excreted. Afterwards I +allowed an ant to visit them, and it immediately seemed, by its eager +way of running about, to be well aware what a rich flock it had +discovered; it then began to play with its antennæ on the abdomen first +of one aphis and then of another; and each aphis, as soon as it felt +the antennæ, immediately lifted up its abdomen and excreted a limpid +drop of sweet juice, which was eagerly devoured by the ant. Even the +quite young aphides behaved in this manner, showing that the action was +instinctive, and not the result of experience. But as the excretion is +extremely viscid, it is probably a convenience to the aphides to have +it removed; and therefore probably the aphides do not instinctively +excrete for the sole good of the ants. Although I do not believe that +any animal in the world performs an action for the exclusive good of +another of a distinct species, yet each species tries to take advantage +of the instincts of others, as each takes advantage of the weaker +bodily structure of others. So again, in some few cases, certain +instincts cannot be considered as absolutely perfect; but as details on +this and other such points are not indispensable, they may be here +passed over. + +As some degree of variation in instincts under a state of nature, and +the inheritance of such variations, are indispensable for the action of +natural selection, as many instances as possible ought to have been +here given; but want of space prevents me. I can only assert, that +instincts certainly do vary—for instance, +the migratory instinct, both in extent and direction, and in its total +loss. So it is with the nests of birds, which vary partly in dependence +on the situations chosen, and on the nature and temperature of the +country inhabited, but often from causes wholly unknown to us: Audubon +has given several remarkable cases of differences in nests of the same +species in the northern and southern United States. Fear of any +particular enemy is certainly an instinctive quality, as may be seen in +nestling birds, though it is strengthened by experience, and by the +sight of fear of the same enemy in other animals. But fear of man is +slowly acquired, as I have elsewhere shown, by various animals +inhabiting desert islands; and we may see an instance of this, even in +England, in the greater wildness of all our large birds than of our +small birds; for the large birds have been most persecuted by man. We +may safely attribute the greater wildness of our large birds to this +cause; for in uninhabited islands large birds are not more fearful than +small; and the magpie, so wary in England, is tame in Norway, as is the +hooded crow in Egypt. + +That the general disposition of individuals of the same species, born +in a state of nature, is extremely diversified, can be shown by a +multitude of facts. Several cases also, could be given, of occasional +and strange habits in certain species, which might, if advantageous to +the species, give rise, through natural selection, to quite new +instincts. But I am well aware that these general statements, without +facts given in detail, can produce but a feeble effect on the reader’s +mind. I can only repeat my assurance, that I do not speak without good +evidence. + +The possibility, or even probability, of inherited variations of +instinct in a state of nature will be strengthened by briefly +considering a few cases under +domestication. We shall thus also be enabled to see the respective +parts which habit and the selection of so-called accidental variations +have played in modifying the mental qualities of our domestic animals. +A number of curious and authentic instances could be given of the +inheritance of all shades of disposition and tastes, and likewise of +the oddest tricks, associated with certain frames of mind or periods of +time. But let us look to the familiar case of the several breeds of +dogs: it cannot be doubted that young pointers (I have myself seen a +striking instance) will sometimes point and even back other dogs the +very first time that they are taken out; retrieving is certainly in +some degree inherited by retrievers; and a tendency to run round, +instead of at, a flock of sheep, by shepherd-dogs. I cannot see that +these actions, performed without experience by the young, and in nearly +the same manner by each individual, performed with eager delight by +each breed, and without the end being known,—for the young pointer can +no more know that he points to aid his master, than the white butterfly +knows why she lays her eggs on the leaf of the cabbage,—I cannot see +that these actions differ essentially from true instincts. If we were +to see one kind of wolf, when young and without any training, as soon +as it scented its prey, stand motionless like a statue, and then slowly +crawl forward with a peculiar gait; and another kind of wolf rushing +round, instead of at, a herd of deer, and driving them to a distant +point, we should assuredly call these actions instinctive. Domestic +instincts, as they may be called, are certainly far less fixed or +invariable than natural instincts; but they have been acted on by far +less rigorous selection, and have been transmitted for an incomparably +shorter period, under less fixed conditions of life. + +How strongly these domestic instincts, habits, and dispositions +are inherited, and how curiously they become mingled, is well shown +when different breeds of dogs are crossed. Thus it is known that a +cross with a bull-dog has affected for many generations the courage and +obstinacy of greyhounds; and a cross with a greyhound has given to a +whole family of shepherd-dogs a tendency to hunt hares. These domestic +instincts, when thus tested by crossing, resemble natural instincts, +which in a like manner become curiously blended together, and for a +long period exhibit traces of the instincts of either parent: for +example, Le Roy describes a dog, whose great-grandfather was a wolf, +and this dog showed a trace of its wild parentage only in one way, by +not coming in a straight line to his master when called. + +Domestic instincts are sometimes spoken of as actions which have become +inherited solely from long-continued and compulsory habit, but this, I +think, is not true. No one would ever have thought of teaching, or +probably could have taught, the tumbler-pigeon to tumble,—an action +which, as I have witnessed, is performed by young birds, that have +never seen a pigeon tumble. We may believe that some one pigeon showed +a slight tendency to this strange habit, and that the long-continued +selection of the best individuals in successive generations made +tumblers what they now are; and near Glasgow there are house-tumblers, +as I hear from Mr. Brent, which cannot fly eighteen inches high without +going head over heels. It may be doubted whether any one would have +thought of training a dog to point, had not some one dog naturally +shown a tendency in this line; and this is known occasionally to +happen, as I once saw in a pure terrier. When the first tendency was +once displayed, methodical selection and the inherited effects of +compulsory training in each successive generation would soon complete +the work; and unconscious +selection is still at work, as each man tries to procure, without +intending to improve the breed, dogs which will stand and hunt best. On +the other hand, habit alone in some cases has sufficed; no animal is +more difficult to tame than the young of the wild rabbit; scarcely any +animal is tamer than the young of the tame rabbit; but I do not suppose +that domestic rabbits have ever been selected for tameness; and I +presume that we must attribute the whole of the inherited change from +extreme wildness to extreme tameness, simply to habit and +long-continued close confinement. + +Natural instincts are lost under domestication: a remarkable instance +of this is seen in those breeds of fowls which very rarely or never +become “broody,” that is, never wish to sit on their eggs. Familiarity +alone prevents our seeing how universally and largely the minds of our +domestic animals have been modified by domestication. It is scarcely +possible to doubt that the love of man has become instinctive in the +dog. All wolves, foxes, jackals, and species of the cat genus, when +kept tame, are most eager to attack poultry, sheep, and pigs; and this +tendency has been found incurable in dogs which have been brought home +as puppies from countries, such as Tierra del Fuego and Australia, +where the savages do not keep these domestic animals. How rarely, on +the other hand, do our civilised dogs, even when quite young, require +to be taught not to attack poultry, sheep, and pigs! No doubt they +occasionally do make an attack, and are then beaten; and if not cured, +they are destroyed; so that habit, with some degree of selection, has +probably concurred in civilising by inheritance our dogs. On the other +hand, young chickens have lost, wholly by habit, that fear of the dog +and cat which no doubt was originally instinctive in them, in the same +way as it is so plainly instinctive in +young pheasants, though reared under a hen. It is not that chickens +have lost all fear, but fear only of dogs and cats, for if the hen +gives the danger-chuckle, they will run (more especially young turkeys) +from under her, and conceal themselves in the surrounding grass or +thickets; and this is evidently done for the instinctive purpose of +allowing, as we see in wild ground-birds, their mother to fly away. But +this instinct retained by our chickens has become useless under +domestication, for the mother-hen has almost lost by disuse the power +of flight. + +Hence, we may conclude, that domestic instincts have been acquired and +natural instincts have been lost partly by habit, and partly by man +selecting and accumulating during successive generations, peculiar +mental habits and actions, which at first appeared from what we must in +our ignorance call an accident. In some cases compulsory habit alone +has sufficed to produce such inherited mental changes; in other cases +compulsory habit has done nothing, and all has been the result of +selection, pursued both methodically and unconsciously; but in most +cases, probably, habit and selection have acted together. + +We shall, perhaps, best understand how instincts in a state of nature +have become modified by selection, by considering a few cases. I will +select only three, out of the several which I shall have to discuss in +my future work,—namely, the instinct which leads the cuckoo to lay her +eggs in other birds’ nests; the slave-making instinct of certain ants; +and the comb-making power of the hive-bee: these two latter instincts +have generally, and most justly, been ranked by naturalists as the most +wonderful of all known instincts. + +It is now commonly admitted that the more immediate and final cause of +the cuckoo’s instinct is, that +she lays her eggs, not daily, but at intervals of two or three days; so +that, if she were to make her own nest and sit on her own eggs, those +first laid would have to be left for some time unincubated, or there +would be eggs and young birds of different ages in the same nest. If +this were the case, the process of laying and hatching might be +inconveniently long, more especially as she has to migrate at a very +early period; and the first hatched young would probably have to be fed +by the male alone. But the American cuckoo is in this predicament; for +she makes her own nest and has eggs and young successively hatched, all +at the same time. It has been asserted that the American cuckoo +occasionally lays her eggs in other birds’ nests; but I hear on the +high authority of Dr. Brewer, that this is a mistake. Nevertheless, I +could give several instances of various birds which have been known +occasionally to lay their eggs in other birds’ nests. Now let us +suppose that the ancient progenitor of our European cuckoo had the +habits of the American cuckoo; but that occasionally she laid an egg in +another bird’s nest. If the old bird profited by this occasional habit, +or if the young were made more vigorous by advantage having been taken +of the mistaken maternal instinct of another bird, than by their own +mother’s care, encumbered as she can hardly fail to be by having eggs +and young of different ages at the same time; then the old birds or the +fostered young would gain an advantage. And analogy would lead me to +believe, that the young thus reared would be apt to follow by +inheritance the occasional and aberrant habit of their mother, and in +their turn would be apt to lay their eggs in other birds’ nests, and +thus be successful in rearing their young. By a continued process of +this nature, I believe that the strange instinct of our cuckoo could +be, and has been, +generated. I may add that, according to Dr. Gray and to some other +observers, the European cuckoo has not utterly lost all maternal love +and care for her own offspring. + +The occasional habit of birds laying their eggs in other birds’ nests, +either of the same or of a distinct species, is not very uncommon with +the Gallinaceæ; and this perhaps explains the origin of a singular +instinct in the allied group of ostriches. For several hen ostriches, +at least in the case of the American species, unite and lay first a few +eggs in one nest and then in another; and these are hatched by the +males. This instinct may probably be accounted for by the fact of the +hens laying a large number of eggs; but, as in the case of the cuckoo, +at intervals of two or three days. This instinct, however, of the +American ostrich has not as yet been perfected; for a surprising number +of eggs lie strewed over the plains, so that in one day’s hunting I +picked up no less than twenty lost and wasted eggs. + +Many bees are parasitic, and always lay their eggs in the nests of bees +of other kinds. This case is more remarkable than that of the cuckoo; +for these bees have not only their instincts but their structure +modified in accordance with their parasitic habits; for they do not +possess the pollen-collecting apparatus which would be necessary if +they had to store food for their own young. Some species, likewise, of +Sphegidæ (wasp-like insects) are parasitic on other species; and M. +Fabre has lately shown good reason for believing that although the +Tachytes nigra generally makes its own burrow and stores it with +paralysed prey for its own larvæ to feed on, yet that when this insect +finds a burrow already made and stored by another sphex, it takes +advantage of the prize, and becomes for the occasion parasitic. In this +case, as with the supposed case of the cuckoo, I can +see no difficulty in natural selection making an occasional habit +permanent, if of advantage to the species, and if the insect whose nest +and stored food are thus feloniously appropriated, be not thus +exterminated. + +_Slave-making instinct_.—This remarkable instinct was first discovered +in the Formica (Polyerges) rufescens by Pierre Huber, a better observer +even than his celebrated father. This ant is absolutely dependent on +its slaves; without their aid, the species would certainly become +extinct in a single year. The males and fertile females do no work. The +workers or sterile females, though most energetic and courageous in +capturing slaves, do no other work. They are incapable of making their +own nests, or of feeding their own larvæ. When the old nest is found +inconvenient, and they have to migrate, it is the slaves which +determine the migration, and actually carry their masters in their +jaws. So utterly helpless are the masters, that when Huber shut up +thirty of them without a slave, but with plenty of the food which they +like best, and with their larvæ and pupæ to stimulate them to work, +they did nothing; they could not even feed themselves, and many +perished of hunger. Huber then introduced a single slave (F. fusca), +and she instantly set to work, fed and saved the survivors; made some +cells and tended the larvæ, and put all to rights. What can be more +extraordinary than these well-ascertained facts? If we had not known of +any other slave-making ant, it would have been hopeless to have +speculated how so wonderful an instinct could have been perfected. + +Formica sanguinea was likewise first discovered by P. Huber to be a +slave-making ant. This species is found in the southern parts of +England, and its habits have been attended to by Mr. F. Smith, of the +British +Museum, to whom I am much indebted for information on this and other +subjects. Although fully trusting to the statements of Huber and Mr. +Smith, I tried to approach the subject in a sceptical frame of mind, as +any one may well be excused for doubting the truth of so extraordinary +and odious an instinct as that of making slaves. Hence I will give the +observations which I have myself made, in some little detail. I opened +fourteen nests of F. sanguinea, and found a few slaves in all. Males +and fertile females of the slave-species are found only in their own +proper communities, and have never been observed in the nests of F. +sanguinea. The slaves are black and not above half the size of their +red masters, so that the contrast in their appearance is very great. +When the nest is slightly disturbed, the slaves occasionally come out, +and like their masters are much agitated and defend the nest: when the +nest is much disturbed and the larvæ and pupæ are exposed, the slaves +work energetically with their masters in carrying them away to a place +of safety. Hence, it is clear, that the slaves feel quite at home. +During the months of June and July, on three successive years, I have +watched for many hours several nests in Surrey and Sussex, and never +saw a slave either leave or enter a nest. As, during these months, the +slaves are very few in number, I thought that they might behave +differently when more numerous; but Mr. Smith informs me that he has +watched the nests at various hours during May, June and August, both in +Surrey and Hampshire, and has never seen the slaves, though present in +large numbers in August, either leave or enter the nest. Hence he +considers them as strictly household slaves. The masters, on the other +hand, may be constantly seen bringing in materials for the nest, and +food of all kinds. During the present year, however, in the month +of July, I came across a community with an unusually large stock of +slaves, and I observed a few slaves mingled with their masters leaving +the nest, and marching along the same road to a tall Scotch-fir-tree, +twenty-five yards distant, which they ascended together, probably in +search of aphides or cocci. According to Huber, who had ample +opportunities for observation, in Switzerland the slaves habitually +work with their masters in making the nest, and they alone open and +close the doors in the morning and evening; and, as Huber expressly +states, their principal office is to search for aphides. This +difference in the usual habits of the masters and slaves in the two +countries, probably depends merely on the slaves being captured in +greater numbers in Switzerland than in England. + +One day I fortunately chanced to witness a migration from one nest to +another, and it was a most interesting spectacle to behold the masters +carefully carrying, as Huber has described, their slaves in their jaws. +Another day my attention was struck by about a score of the +slave-makers haunting the same spot, and evidently not in search of +food; they approached and were vigorously repulsed by an independent +community of the slave species (F. fusca); sometimes as many as three +of these ants clinging to the legs of the slave-making F. sanguinea. +The latter ruthlessly killed their small opponents, and carried their +dead bodies as food to their nest, twenty-nine yards distant; but they +were prevented from getting any pupæ to rear as slaves. I then dug up a +small parcel of the pupæ of F. fusca from another nest, and put them +down on a bare spot near the place of combat; they were eagerly seized, +and carried off by the tyrants, who perhaps fancied that, after all, +they had been victorious in their late combat. + + +At the same time I laid on the same place a small parcel of the pupæ of +another species, F. flava, with a few of these little yellow ants still +clinging to the fragments of the nest. This species is sometimes, +though rarely, made into slaves, as has been described by Mr. Smith. +Although so small a species, it is very courageous, and I have seen it +ferociously attack other ants. In one instance I found to my surprise +an independent community of F. flava under a stone beneath a nest of +the slave-making F. sanguinea; and when I had accidentally disturbed +both nests, the little ants attacked their big neighbours with +surprising courage. Now I was curious to ascertain whether F. sanguinea +could distinguish the pupæ of F. fusca, which they habitually make into +slaves, from those of the little and furious F. flava, which they +rarely capture, and it was evident that they did at once distinguish +them: for we have seen that they eagerly and instantly seized the pupæ +of F. fusca, whereas they were much terrified when they came across the +pupæ, or even the earth from the nest of F. flava, and quickly ran +away; but in about a quarter of an hour, shortly after all the little +yellow ants had crawled away, they took heart and carried off the pupæ. + +One evening I visited another community of F. sanguinea, and found a +number of these ants entering their nest, carrying the dead bodies of +F. fusca (showing that it was not a migration) and numerous pupæ. I +traced the returning file burthened with booty, for about forty yards, +to a very thick clump of heath, whence I saw the last individual of F. +sanguinea emerge, carrying a pupa; but I was not able to find the +desolated nest in the thick heath. The nest, however, must have been +close at hand, for two or three individuals of F. fusca were rushing +about in the greatest agitation, and one was +perched motionless with its own pupa in its mouth on the top of a spray +of heath over its ravaged home. + +Such are the facts, though they did not need confirmation by me, in +regard to the wonderful instinct of making slaves. Let it be observed +what a contrast the instinctive habits of F. sanguinea present with +those of the F. rufescens. The latter does not build its own nest, does +not determine its own migrations, does not collect food for itself or +its young, and cannot even feed itself: it is absolutely dependent on +its numerous slaves. Formica sanguinea, on the other hand, possesses +much fewer slaves, and in the early part of the summer extremely few. +The masters determine when and where a new nest shall be formed, and +when they migrate, the masters carry the slaves. Both in Switzerland +and England the slaves seem to have the exclusive care of the larvæ, +and the masters alone go on slave-making expeditions. In Switzerland +the slaves and masters work together, making and bringing materials for +the nest: both, but chiefly the slaves, tend, and milk as it may be +called, their aphides; and thus both collect food for the community. In +England the masters alone usually leave the nest to collect building +materials and food for themselves, their slaves and larvæ. So that the +masters in this country receive much less service from their slaves +than they do in Switzerland. + +By what steps the instinct of F. sanguinea originated I will not +pretend to conjecture. But as ants, which are not slave-makers, will, +as I have seen, carry off pupæ of other species, if scattered near +their nests, it is possible that pupæ originally stored as food might +become developed; and the ants thus unintentionally reared would then +follow their proper instincts, and do what work they could. If their +presence proved useful to the species which had seized them—if it were +more advantageous +to this species to capture workers than to procreate them—the habit of +collecting pupæ originally for food might by natural selection be +strengthened and rendered permanent for the very different purpose of +raising slaves. When the instinct was once acquired, if carried out to +a much less extent even than in our British F. sanguinea, which, as we +have seen, is less aided by its slaves than the same species in +Switzerland, I can see no difficulty in natural selection increasing +and modifying the instinct—always supposing each modification to be of +use to the species—until an ant was formed as abjectly dependent on its +slaves as is the Formica rufescens. + +_Cell-making instinct of the Hive-Bee_.—I will not here enter on minute +details on this subject, but will merely give an outline of the +conclusions at which I have arrived. He must be a dull man who can +examine the exquisite structure of a comb, so beautifully adapted to +its end, without enthusiastic admiration. We hear from mathematicians +that bees have practically solved a recondite problem, and have made +their cells of the proper shape to hold the greatest possible amount of +honey, with the least possible consumption of precious wax in their +construction. It has been remarked that a skilful workman, with fitting +tools and measures, would find it very difficult to make cells of wax +of the true form, though this is perfectly effected by a crowd of bees +working in a dark hive. Grant whatever instincts you please, and it +seems at first quite inconceivable how they can make all the necessary +angles and planes, or even perceive when they are correctly made. But +the difficulty is not nearly so great as it at first appears: all this +beautiful work can be shown, I think, to follow from a few very simple +instincts. + + +I was led to investigate this subject by Mr. Waterhouse, who has shown +that the form of the cell stands in close relation to the presence of +adjoining cells; and the following view may, perhaps, be considered +only as a modification of his theory. Let us look to the great +principle of gradation, and see whether Nature does not reveal to us +her method of work. At one end of a short series we have humble-bees, +which use their old cocoons to hold honey, sometimes adding to them +short tubes of wax, and likewise making separate and very irregular +rounded cells of wax. At the other end of the series we have the cells +of the hive-bee, placed in a double layer: each cell, as is well known, +is an hexagonal prism, with the basal edges of its six sides bevelled +so as to join on to a pyramid, formed of three rhombs. These rhombs +have certain angles, and the three which form the pyramidal base of a +single cell on one side of the comb, enter into the composition of the +bases of three adjoining cells on the opposite side. In the series +between the extreme perfection of the cells of the hive-bee and the +simplicity of those of the humble-bee, we have the cells of the Mexican +Melipona domestica, carefully described and figured by Pierre Huber. +The Melipona itself is intermediate in structure between the hive and +humble bee, but more nearly related to the latter: it forms a nearly +regular waxen comb of cylindrical cells, in which the young are +hatched, and, in addition, some large cells of wax for holding honey. +These latter cells are nearly spherical and of nearly equal sizes, and +are aggregated into an irregular mass. But the important point to +notice, is that these cells are always made at that degree of nearness +to each other, that they would have intersected or broken into each +other, if the spheres had been completed; but this is never permitted, +the bees building perfectly flat walls of wax between the spheres +which thus tend to intersect. Hence each cell consists of an outer +spherical portion and of two, three, or more perfectly flat surfaces, +according as the cell adjoins two, three or more other cells. When one +cell comes into contact with three other cells, which, from the spheres +being nearly of the same size, is very frequently and necessarily the +case, the three flat surfaces are united into a pyramid; and this +pyramid, as Huber has remarked, is manifestly a gross imitation of the +three-sided pyramidal basis of the cell of the hive-bee. As in the +cells of the hive-bee, so here, the three plane surfaces in any one +cell necessarily enter into the construction of three adjoining cells. +It is obvious that the Melipona saves wax by this manner of building; +for the flat walls between the adjoining cells are not double, but are +of the same thickness as the outer spherical portions, and yet each +flat portion forms a part of two cells. + +Reflecting on this case, it occurred to me that if the Melipona had +made its spheres at some given distance from each other, and had made +them of equal sizes and had arranged them symmetrically in a double +layer, the resulting structure would probably have been as perfect as +the comb of the hive-bee. Accordingly I wrote to Professor Miller, of +Cambridge, and this geometer has kindly read over the following +statement, drawn up from his information, and tells me that it is +strictly correct:— + +If a number of equal spheres be described with their centres placed in +two parallel layers; with the centre of each sphere at the distance of +radius x the square root of 2 or radius x 1.41421 (or at some lesser +distance), from the centres of the six surrounding spheres in the same +layer; and at the same distance from the centres of the adjoining +spheres in the other and parallel layer; then, if planes of +intersection between the several spheres in +both layers be formed, there will result a double layer of hexagonal +prisms united together by pyramidal bases formed of three rhombs; and +the rhombs and the sides of the hexagonal prisms will have every angle +identically the same with the best measurements which have been made of +the cells of the hive-bee. + +Hence we may safely conclude that if we could slightly modify the +instincts already possessed by the Melipona, and in themselves not very +wonderful, this bee would make a structure as wonderfully perfect as +that of the hive-bee. We must suppose the Melipona to make her cells +truly spherical, and of equal sizes; and this would not be very +surprising, seeing that she already does so to a certain extent, and +seeing what perfectly cylindrical burrows in wood many insects can +make, apparently by turning round on a fixed point. We must suppose the +Melipona to arrange her cells in level layers, as she already does her +cylindrical cells; and we must further suppose, and this is the +greatest difficulty, that she can somehow judge accurately at what +distance to stand from her fellow-labourers when several are making +their spheres; but she is already so far enabled to judge of distance, +that she always describes her spheres so as to intersect largely; and +then she unites the points of intersection by perfectly flat surfaces. +We have further to suppose, but this is no difficulty, that after +hexagonal prisms have been formed by the intersection of adjoining +spheres in the same layer, she can prolong the hexagon to any length +requisite to hold the stock of honey; in the same way as the rude +humble-bee adds cylinders of wax to the circular mouths of her old +cocoons. By such modifications of instincts in themselves not very +wonderful,—hardly more wonderful than those which guide a bird to make +its nest,—I believe that the hive-bee +has acquired, through natural selection, her inimitable architectural +powers. + +But this theory can be tested by experiment. Following the example of +Mr. Tegetmeier, I separated two combs, and put between them a long, +thick, square strip of wax: the bees instantly began to excavate minute +circular pits in it; and as they deepened these little pits, they made +them wider and wider until they were converted into shallow basins, +appearing to the eye perfectly true or parts of a sphere, and of about +the diameter of a cell. It was most interesting to me to observe that +wherever several bees had begun to excavate these basins near together, +they had begun their work at such a distance from each other, that by +the time the basins had acquired the above stated width (_i.e._ about +the width of an ordinary cell), and were in depth about one sixth of +the diameter of the sphere of which they formed a part, the rims of the +basins intersected or broke into each other. As soon as this occurred, +the bees ceased to excavate, and began to build up flat walls of wax on +the lines of intersection between the basins, so that each hexagonal +prism was built upon the festooned edge of a smooth basin, instead of +on the straight edges of a three-sided pyramid as in the case of +ordinary cells. + +I then put into the hive, instead of a thick, square piece of wax, a +thin and narrow, knife-edged ridge, coloured with vermilion. The bees +instantly began on both sides to excavate little basins near to each +other, in the same way as before; but the ridge of wax was so thin, +that the bottoms of the basins, if they had been excavated to the same +depth as in the former experiment, would have broken into each other +from the opposite sides. The bees, however, did not suffer this to +happen, and they stopped their excavations in due +time; so that the basins, as soon as they had been a little deepened, +came to have flat bottoms; and these flat bottoms, formed by thin +little plates of the vermilion wax having been left ungnawed, were +situated, as far as the eye could judge, exactly along the planes of +imaginary intersection between the basins on the opposite sides of the +ridge of wax. In parts, only little bits, in other parts, large +portions of a rhombic plate had been left between the opposed basins, +but the work, from the unnatural state of things, had not been neatly +performed. The bees must have worked at very nearly the same rate on +the opposite sides of the ridge of vermilion wax, as they circularly +gnawed away and deepened the basins on both sides, in order to have +succeeded in thus leaving flat plates between the basins, by stopping +work along the intermediate planes or planes of intersection. + +Considering how flexible thin wax is, I do not see that there is any +difficulty in the bees, whilst at work on the two sides of a strip of +wax, perceiving when they have gnawed the wax away to the proper +thinness, and then stopping their work. In ordinary combs it has +appeared to me that the bees do not always succeed in working at +exactly the same rate from the opposite sides; for I have noticed +half-completed rhombs at the base of a just-commenced cell, which were +slightly concave on one side, where I suppose that the bees had +excavated too quickly, and convex on the opposed side, where the bees +had worked less quickly. In one well-marked instance, I put the comb +back into the hive, and allowed the bees to go on working for a short +time, and again examined the cell, and I found that the rhombic plate +had been completed, and had become _perfectly flat:_ it was absolutely +impossible, from the extreme thinness of the little rhombic plate, that +they could have effected +this by gnawing away the convex side; and I suspect that the bees in +such cases stand in the opposed cells and push and bend the ductile and +warm wax (which as I have tried is easily done) into its proper +intermediate plane, and thus flatten it. + +From the experiment of the ridge of vermilion wax, we can clearly see +that if the bees were to build for themselves a thin wall of wax, they +could make their cells of the proper shape, by standing at the proper +distance from each other, by excavating at the same rate, and by +endeavouring to make equal spherical hollows, but never allowing the +spheres to break into each other. Now bees, as may be clearly seen by +examining the edge of a growing comb, do make a rough, circumferential +wall or rim all round the comb; and they gnaw into this from the +opposite sides, always working circularly as they deepen each cell. +They do not make the whole three-sided pyramidal base of any one cell +at the same time, but only the one rhombic plate which stands on the +extreme growing margin, or the two plates, as the case may be; and they +never complete the upper edges of the rhombic plates, until the +hexagonal walls are commenced. Some of these statements differ from +those made by the justly celebrated elder Huber, but I am convinced of +their accuracy; and if I had space, I could show that they are +conformable with my theory. + +Huber’s statement that the very first cell is excavated out of a little +parallel-sided wall of wax, is not, as far as I have seen, strictly +correct; the first commencement having always been a little hood of +wax; but I will not here enter on these details. We see how important a +part excavation plays in the construction of the cells; but it would be +a great error to suppose that the bees cannot build up a rough wall of +wax in the proper +position—that is, along the plane of intersection between two adjoining +spheres. I have several specimens showing clearly that they can do +this. Even in the rude circumferential rim or wall of wax round a +growing comb, flexures may sometimes be observed, corresponding in +position to the planes of the rhombic basal plates of future cells. But +the rough wall of wax has in every case to be finished off, by being +largely gnawed away on both sides. The manner in which the bees build +is curious; they always make the first rough wall from ten to twenty +times thicker than the excessively thin finished wall of the cell, +which will ultimately be left. We shall understand how they work, by +supposing masons first to pile up a broad ridge of cement, and then to +begin cutting it away equally on both sides near the ground, till a +smooth, very thin wall is left in the middle; the masons always piling +up the cut-away cement, and adding fresh cement, on the summit of the +ridge. We shall thus have a thin wall steadily growing upward; but +always crowned by a gigantic coping. From all the cells, both those +just commenced and those completed, being thus crowned by a strong +coping of wax, the bees can cluster and crawl over the comb without +injuring the delicate hexagonal walls, which are only about one +four-hundredth of an inch in thickness; the plates of the pyramidal +basis being about twice as thick. By this singular manner of building, +strength is continually given to the comb, with the utmost ultimate +economy of wax. + +It seems at first to add to the difficulty of understanding how the +cells are made, that a multitude of bees all work together; one bee +after working a short time at one cell going to another, so that, as +Huber has stated, a score of individuals work even at the commencement +of the first cell. I was able practically to show this fact, by +covering the edges of the hexagonal walls +of a single cell, or the extreme margin of the circumferential rim of a +growing comb, with an extremely thin layer of melted vermilion wax; and +I invariably found that the colour was most delicately diffused by the +bees—as delicately as a painter could have done with his brush—by atoms +of the coloured wax having been taken from the spot on which it had +been placed, and worked into the growing edges of the cells all round. +The work of construction seems to be a sort of balance struck between +many bees, all instinctively standing at the same relative distance +from each other, all trying to sweep equal spheres, and then building +up, or leaving ungnawed, the planes of intersection between these +spheres. It was really curious to note in cases of difficulty, as when +two pieces of comb met at an angle, how often the bees would entirely +pull down and rebuild in different ways the same cell, sometimes +recurring to a shape which they had at first rejected. + +When bees have a place on which they can stand in their proper +positions for working,—for instance, on a slip of wood, placed directly +under the middle of a comb growing downwards so that the comb has to be +built over one face of the slip—in this case the bees can lay the +foundations of one wall of a new hexagon, in its strictly proper place, +projecting beyond the other completed cells. It suffices that the bees +should be enabled to stand at their proper relative distances from each +other and from the walls of the last completed cells, and then, by +striking imaginary spheres, they can build up a wall intermediate +between two adjoining spheres; but, as far as I have seen, they never +gnaw away and finish off the angles of a cell till a large part both of +that cell and of the adjoining cells has been built. This capacity in +bees of laying down under certain circumstances a rough wall in its +proper place between two just-commenced +cells, is important, as it bears on a fact, which seems at first quite +subversive of the foregoing theory; namely, that the cells on the +extreme margin of wasp-combs are sometimes strictly hexagonal; but I +have not space here to enter on this subject. Nor does there seem to me +any great difficulty in a single insect (as in the case of a +queen-wasp) making hexagonal cells, if she work alternately on the +inside and outside of two or three cells commenced at the same time, +always standing at the proper relative distance from the parts of the +cells just begun, sweeping spheres or cylinders, and building up +intermediate planes. It is even conceivable that an insect might, by +fixing on a point at which to commence a cell, and then moving outside, +first to one point, and then to five other points, at the proper +relative distances from the central point and from each other, strike +the planes of intersection, and so make an isolated hexagon: but I am +not aware that any such case has been observed; nor would any good be +derived from a single hexagon being built, as in its construction more +materials would be required than for a cylinder. + +As natural selection acts only by the accumulation of slight +modifications of structure or instinct, each profitable to the +individual under its conditions of life, it may reasonably be asked, +how a long and graduated succession of modified architectural +instincts, all tending towards the present perfect plan of +construction, could have profited the progenitors of the hive-bee? I +think the answer is not difficult: it is known that bees are often hard +pressed to get sufficient nectar; and I am informed by Mr. Tegetmeier +that it has been experimentally found that no less than from twelve to +fifteen pounds of dry sugar are consumed by a hive of bees for the +secretion of each pound of wax; so that a prodigious quantity of fluid +nectar must be collected and consumed by the bees in a hive for +the secretion of the wax necessary for the construction of their combs. +Moreover, many bees have to remain idle for many days during the +process of secretion. A large store of honey is indispensable to +support a large stock of bees during the winter; and the security of +the hive is known mainly to depend on a large number of bees being +supported. Hence the saving of wax by largely saving honey must be a +most important element of success in any family of bees. Of course the +success of any species of bee may be dependent on the number of its +parasites or other enemies, or on quite distinct causes, and so be +altogether independent of the quantity of honey which the bees could +collect. But let us suppose that this latter circumstance determined, +as it probably often does determine, the numbers of a humble-bee which +could exist in a country; and let us further suppose that the community +lived throughout the winter, and consequently required a store of +honey: there can in this case be no doubt that it would be an advantage +to our humble-bee, if a slight modification of her instinct led her to +make her waxen cells near together, so as to intersect a little; for a +wall in common even to two adjoining cells, would save some little wax. +Hence it would continually be more and more advantageous to our +humble-bee, if she were to make her cells more and more regular, nearer +together, and aggregated into a mass, like the cells of the Melipona; +for in this case a large part of the bounding surface of each cell +would serve to bound other cells, and much wax would be saved. Again, +from the same cause, it would be advantageous to the Melipona, if she +were to make her cells closer together, and more regular in every way +than at present; for then, as we have seen, the spherical surfaces +would wholly disappear, and would all be replaced by plane surfaces; +and the Melipona +would make a comb as perfect as that of the hive-bee. Beyond this stage +of perfection in architecture, natural selection could not lead; for +the comb of the hive-bee, as far as we can see, is absolutely perfect +in economising wax. + +Thus, as I believe, the most wonderful of all known instincts, that of +the hive-bee, can be explained by natural selection having taken +advantage of numerous, successive, slight modifications of simpler +instincts; natural selection having by slow degrees, more and more +perfectly, led the bees to sweep equal spheres at a given distance from +each other in a double layer, and to build up and excavate the wax +along the planes of intersection. The bees, of course, no more knowing +that they swept their spheres at one particular distance from each +other, than they know what are the several angles of the hexagonal +prisms and of the basal rhombic plates. The motive power of the process +of natural selection having been economy of wax; that individual swarm +which wasted least honey in the secretion of wax, having succeeded +best, and having transmitted by inheritance its newly acquired +economical instinct to new swarms, which in their turn will have had +the best chance of succeeding in the struggle for existence. + +No doubt many instincts of very difficult explanation could be opposed +to the theory of natural selection,—cases, in which we cannot see how +an instinct could possibly have originated; cases, in which no +intermediate gradations are known to exist; cases of instinct of +apparently such trifling importance, that they could hardly have been +acted on by natural selection; cases of instincts almost identically +the same in animals so remote in the scale of nature, that we cannot +account +for their similarity by inheritance from a common parent, and must +therefore believe that they have been acquired by independent acts of +natural selection. I will not here enter on these several cases, but +will confine myself to one special difficulty, which at first appeared +to me insuperable, and actually fatal to my whole theory. I allude to +the neuters or sterile females in insect-communities: for these neuters +often differ widely in instinct and in structure from both the males +and fertile females, and yet, from being sterile, they cannot propagate +their kind. + +The subject well deserves to be discussed at great length, but I will +here take only a single case, that of working or sterile ants. How the +workers have been rendered sterile is a difficulty; but not much +greater than that of any other striking modification of structure; for +it can be shown that some insects and other articulate animals in a +state of nature occasionally become sterile; and if such insects had +been social, and it had been profitable to the community that a number +should have been annually born capable of work, but incapable of +procreation, I can see no very great difficulty in this being effected +by natural selection. But I must pass over this preliminary difficulty. +The great difficulty lies in the working ants differing widely from +both the males and the fertile females in structure, as in the shape of +the thorax and in being destitute of wings and sometimes of eyes, and +in instinct. As far as instinct alone is concerned, the prodigious +difference in this respect between the workers and the perfect females, +would have been far better exemplified by the hive-bee. If a working +ant or other neuter insect had been an animal in the ordinary state, I +should have unhesitatingly assumed that all its characters had been +slowly acquired through natural selection; namely, by an individual +having been born with some slight profitable modification of structure, +this being inherited by its offspring, which again varied and were +again selected, and so onwards. But with the working ant we have an +insect differing greatly from its parents, yet absolutely sterile; so +that it could never have transmitted successively acquired +modifications of structure or instinct to its progeny. It may well be +asked how is it possible to reconcile this case with the theory of +natural selection? + +First, let it be remembered that we have innumerable instances, both in +our domestic productions and in those in a state of nature, of all +sorts of differences of structure which have become correlated to +certain ages, and to either sex. We have differences correlated not +only to one sex, but to that short period alone when the reproductive +system is active, as in the nuptial plumage of many birds, and in the +hooked jaws of the male salmon. We have even slight differences in the +horns of different breeds of cattle in relation to an artificially +imperfect state of the male sex; for oxen of certain breeds have longer +horns than in other breeds, in comparison with the horns of the bulls +or cows of these same breeds. Hence I can see no real difficulty in any +character having become correlated with the sterile condition of +certain members of insect-communities: the difficulty lies in +understanding how such correlated modifications of structure could have +been slowly accumulated by natural selection. + +This difficulty, though appearing insuperable, is lessened, or, as I +believe, disappears, when it is remembered that selection may be +applied to the family, as well as to the individual, and may thus gain +the desired end. Thus, a well-flavoured vegetable is cooked, and the +individual is destroyed; but the horticulturist sows seeds of the same +stock, and confidently expects to +get nearly the same variety; breeders of cattle wish the flesh and fat +to be well marbled together; the animal has been slaughtered, but the +breeder goes with confidence to the same family. I have such faith in +the powers of selection, that I do not doubt that a breed of cattle, +always yielding oxen with extraordinarily long horns, could be slowly +formed by carefully watching which individual bulls and cows, when +matched, produced oxen with the longest horns; and yet no one ox could +ever have propagated its kind. Thus I believe it has been with social +insects: a slight modification of structure, or instinct, correlated +with the sterile condition of certain members of the community, has +been advantageous to the community: consequently the fertile males and +females of the same community flourished, and transmitted to their +fertile offspring a tendency to produce sterile members having the same +modification. And I believe that this process has been repeated, until +that prodigious amount of difference between the fertile and sterile +females of the same species has been produced, which we see in many +social insects. + +But we have not as yet touched on the climax of the difficulty; namely, +the fact that the neuters of several ants differ, not only from the +fertile females and males, but from each other, sometimes to an almost +incredible degree, and are thus divided into two or even three castes. +The castes, moreover, do not generally graduate into each other, but +are perfectly well defined; being as distinct from each other, as are +any two species of the same genus, or rather as any two genera of the +same family. Thus in Eciton, there are working and soldier neuters, +with jaws and instincts extraordinarily different: in Cryptocerus, the +workers of one caste alone carry a wonderful sort of shield on their +heads, the use of which is quite unknown: in the Mexican Myrmecocystus, +the workers of one caste never leave the nest; they are fed by the +workers of another caste, and they have an enormously developed abdomen +which secretes a sort of honey, supplying the place of that excreted by +the aphides, or the domestic cattle as they may be called, which our +European ants guard or imprison. + +It will indeed be thought that I have an overweening confidence in the +principle of natural selection, when I do not admit that such wonderful +and well-established facts at once annihilate my theory. In the simpler +case of neuter insects all of one caste or of the same kind, which have +been rendered by natural selection, as I believe to be quite possible, +different from the fertile males and females,—in this case, we may +safely conclude from the analogy of ordinary variations, that each +successive, slight, profitable modification did not probably at first +appear in all the individual neuters in the same nest, but in a few +alone; and that by the long-continued selection of the fertile parents +which produced most neuters with the profitable modification, all the +neuters ultimately came to have the desired character. On this view we +ought occasionally to find neuter-insects of the same species, in the +same nest, presenting gradations of structure; and this we do find, +even often, considering how few neuter-insects out of Europe have been +carefully examined. Mr. F. Smith has shown how surprisingly the neuters +of several British ants differ from each other in size and sometimes in +colour; and that the extreme forms can sometimes be perfectly linked +together by individuals taken out of the same nest: I have myself +compared perfect gradations of this kind. It often happens that the +larger or the smaller sized workers are the most numerous; or that both +large and small are numerous, with those of an intermediate size scanty +in numbers. Formica flava has larger and +smaller workers, with some of intermediate size; and, in this species, +as Mr. F. Smith has observed, the larger workers have simple eyes +(ocelli), which though small can be plainly distinguished, whereas the +smaller workers have their ocelli rudimentary. Having carefully +dissected several specimens of these workers, I can affirm that the +eyes are far more rudimentary in the smaller workers than can be +accounted for merely by their proportionally lesser size; and I fully +believe, though I dare not assert so positively, that the workers of +intermediate size have their ocelli in an exactly intermediate +condition. So that we here have two bodies of sterile workers in the +same nest, differing not only in size, but in their organs of vision, +yet connected by some few members in an intermediate condition. I may +digress by adding, that if the smaller workers had been the most useful +to the community, and those males and females had been continually +selected, which produced more and more of the smaller workers, until +all the workers had come to be in this condition; we should then have +had a species of ant with neuters very nearly in the same condition +with those of Myrmica. For the workers of Myrmica have not even +rudiments of ocelli, though the male and female ants of this genus have +well-developed ocelli. + +I may give one other case: so confidently did I expect to find +gradations in important points of structure between the different +castes of neuters in the same species, that I gladly availed myself of +Mr. F. Smith’s offer of numerous specimens from the same nest of the +driver ant (Anomma) of West Africa. The reader will perhaps best +appreciate the amount of difference in these workers, by my giving not +the actual measurements, but a strictly accurate illustration: the +difference was the same as if we were to see a set of workmen building +a house of whom many were five feet four inches high, and many sixteen +feet high; but we must suppose that the larger workmen had heads four +instead of three times as big as those of the smaller men, and jaws +nearly five times as big. The jaws, moreover, of the working ants of +the several sizes differed wonderfully in shape, and in the form and +number of the teeth. But the important fact for us is, that though the +workers can be grouped into castes of different sizes, yet they +graduate insensibly into each other, as does the widely-different +structure of their jaws. I speak confidently on this latter point, as +Mr. Lubbock made drawings for me with the camera lucida of the jaws +which I had dissected from the workers of the several sizes. + +With these facts before me, I believe that natural selection, by acting +on the fertile parents, could form a species which should regularly +produce neuters, either all of large size with one form of jaw, or all +of small size with jaws having a widely different structure; or lastly, +and this is our climax of difficulty, one set of workers of one size +and structure, and simultaneously another set of workers of a different +size and structure;—a graduated series having been first formed, as in +the case of the driver ant, and then the extreme forms, from being the +most useful to the community, having been produced in greater and +greater numbers through the natural selection of the parents which +generated them; until none with an intermediate structure were +produced. + +Thus, as I believe, the wonderful fact of two distinctly defined castes +of sterile workers existing in the same nest, both widely different +from each other and from their parents, has originated. We can see how +useful their production may have been to a social community of insects, +on the same principle that the division of +labour is useful to civilised man. As ants work by inherited instincts +and by inherited tools or weapons, and not by acquired knowledge and +manufactured instruments, a perfect division of labour could be +effected with them only by the workers being sterile; for had they been +fertile, they would have intercrossed, and their instincts and +structure would have become blended. And nature has, as I believe, +effected this admirable division of labour in the communities of ants, +by the means of natural selection. But I am bound to confess, that, +with all my faith in this principle, I should never have anticipated +that natural selection could have been efficient in so high a degree, +had not the case of these neuter insects convinced me of the fact. I +have, therefore, discussed this case, at some little but wholly +insufficient length, in order to show the power of natural selection, +and likewise because this is by far the most serious special +difficulty, which my theory has encountered. The case, also, is very +interesting, as it proves that with animals, as with plants, any amount +of modification in structure can be effected by the accumulation of +numerous, slight, and as we must call them accidental, variations, +which are in any manner profitable, without exercise or habit having +come into play. For no amount of exercise, or habit, or volition, in +the utterly sterile members of a community could possibly have affected +the structure or instincts of the fertile members, which alone leave +descendants. I am surprised that no one has advanced this demonstrative +case of neuter insects, against the well-known doctrine of Lamarck. + +_Summary_.—I have endeavoured briefly in this chapter to show that the +mental qualities of our domestic animals vary, and that the variations +are inherited. Still more briefly I have attempted to show that +instincts +vary slightly in a state of nature. No one will dispute that instincts +are of the highest importance to each animal. Therefore I can see no +difficulty, under changing conditions of life, in natural selection +accumulating slight modifications of instinct to any extent, in any +useful direction. In some cases habit or use and disuse have probably +come into play. I do not pretend that the facts given in this chapter +strengthen in any great degree my theory; but none of the cases of +difficulty, to the best of my judgment, annihilate it. On the other +hand, the fact that instincts are not always absolutely perfect and are +liable to mistakes;—that no instinct has been produced for the +exclusive good of other animals, but that each animal takes advantage +of the instincts of others;—that the canon in natural history, of +“natura non facit saltum” is applicable to instincts as well as to +corporeal structure, and is plainly explicable on the foregoing views, +but is otherwise inexplicable,—all tend to corroborate the theory of +natural selection. + +This theory is, also, strengthened by some few other facts in regard to +instincts; as by that common case of closely allied, but certainly +distinct, species, when inhabiting distant parts of the world and +living under considerably different conditions of life, yet often +retaining nearly the same instincts. For instance, we can understand on +the principle of inheritance, how it is that the thrush of South +America lines its nest with mud, in the same peculiar manner as does +our British thrush: how it is that the male wrens (Troglodytes) of +North America, build “cock-nests,” to roost in, like the males of our +distinct Kitty-wrens,—a habit wholly unlike that of any other known +bird. Finally, it may not be a logical deduction, but to my imagination +it is far more satisfactory to look at such instincts as the young +cuckoo ejecting its foster-brothers,—ants making slaves,—the larvæ of +ichneumonidæ feeding within the live bodies of caterpillars,—not as +specially endowed or created instincts, but as small consequences of +one general law, leading to the advancement of all organic beings, +namely, multiply, vary, let the strongest live and the weakest die. + + + + +CHAPTER VIII. +HYBRIDISM. + + +Distinction between the sterility of first crosses and of hybrids. +Sterility various in degree, not universal, affected by close +interbreeding, removed by domestication. Laws governing the sterility +of hybrids. Sterility not a special endowment, but incidental on other +differences. Causes of the sterility of first crosses and of hybrids. +Parallelism between the effects of changed conditions of life and +crossing. Fertility of varieties when crossed and of their mongrel +offspring not universal. Hybrids and mongrels compared independently of +their fertility. Summary. + + +The view generally entertained by naturalists is that species, when +intercrossed, have been specially endowed with the quality of +sterility, in order to prevent the confusion of all organic forms. This +view certainly seems at first probable, for species within the same +country could hardly have kept distinct had they been capable of +crossing freely. The importance of the fact that hybrids are very +generally sterile, has, I think, been much underrated by some late +writers. On the theory of natural selection the case is especially +important, inasmuch as the sterility of hybrids could not possibly be +of any advantage to them, and therefore could not have been acquired by +the continued preservation of successive profitable degrees of +sterility. I hope, however, to be able to show that sterility is not a +specially acquired or endowed quality, but is incidental on other +acquired differences. + +In treating this subject, two classes of facts, to a large extent +fundamentally different, have generally been confounded together; +namely, the sterility of two +species when first crossed, and the sterility of the hybrids produced +from them. + +Pure species have of course their organs of reproduction in a perfect +condition, yet when intercrossed they produce either few or no +offspring. Hybrids, on the other hand, have their reproductive organs +functionally impotent, as may be clearly seen in the state of the male +element in both plants and animals; though the organs themselves are +perfect in structure, as far as the microscope reveals. In the first +case the two sexual elements which go to form the embryo are perfect; +in the second case they are either not at all developed, or are +imperfectly developed. This distinction is important, when the cause of +the sterility, which is common to the two cases, has to be considered. +The distinction has probably been slurred over, owing to the sterility +in both cases being looked on as a special endowment, beyond the +province of our reasoning powers. + +The fertility of varieties, that is of the forms known or believed to +have descended from common parents, when intercrossed, and likewise the +fertility of their mongrel offspring, is, on my theory, of equal +importance with the sterility of species; for it seems to make a broad +and clear distinction between varieties and species. + +First, for the sterility of species when crossed and of their hybrid +offspring. It is impossible to study the several memoirs and works of +those two conscientious and admirable observers, Kölreuter and Gärtner, +who almost devoted their lives to this subject, without being deeply +impressed with the high generality of some degree of sterility. +Kölreuter makes the rule universal; but then he cuts the knot, for in +ten cases in which he found two forms, considered by most authors as +distinct species, quite fertile together, he +unhesitatingly ranks them as varieties. Gärtner, also, makes the rule +equally universal; and he disputes the entire fertility of Kölreuter’s +ten cases. But in these and in many other cases, Gärtner is obliged +carefully to count the seeds, in order to show that there is any degree +of sterility. He always compares the maximum number of seeds produced +by two species when crossed and by their hybrid offspring, with the +average number produced by both pure parent-species in a state of +nature. But a serious cause of error seems to me to be here introduced: +a plant to be hybridised must be castrated, and, what is often more +important, must be secluded in order to prevent pollen being brought to +it by insects from other plants. Nearly all the plants experimentised +on by Gärtner were potted, and apparently were kept in a chamber in his +house. That these processes are often injurious to the fertility of a +plant cannot be doubted; for Gärtner gives in his table about a score +of cases of plants which he castrated, and artificially fertilised with +their own pollen, and (excluding all cases such as the Leguminosæ, in +which there is an acknowledged difficulty in the manipulation) half of +these twenty plants had their fertility in some degree impaired. +Moreover, as Gärtner during several years repeatedly crossed the +primrose and cowslip, which we have such good reason to believe to be +varieties, and only once or twice succeeded in getting fertile seed; as +he found the common red and blue pimpernels (Anagallis arvensis and +coerulea), which the best botanists rank as varieties, absolutely +sterile together; and as he came to the same conclusion in several +other analogous cases; it seems to me that we may well be permitted to +doubt whether many other species are really so sterile, when +intercrossed, as Gärtner believes. + + +It is certain, on the one hand, that the sterility of various species +when crossed is so different in degree and graduates away so +insensibly, and, on the other hand, that the fertility of pure species +is so easily affected by various circumstances, that for all practical +purposes it is most difficult to say where perfect fertility ends and +sterility begins. I think no better evidence of this can be required +than that the two most experienced observers who have ever lived, +namely, Kölreuter and Gärtner, should have arrived at diametrically +opposite conclusions in regard to the very same species. It is also +most instructive to compare—but I have not space here to enter on +details—the evidence advanced by our best botanists on the question +whether certain doubtful forms should be ranked as species or +varieties, with the evidence from fertility adduced by different +hybridisers, or by the same author, from experiments made during +different years. It can thus be shown that neither sterility nor +fertility affords any clear distinction between species and varieties; +but that the evidence from this source graduates away, and is doubtful +in the same degree as is the evidence derived from other constitutional +and structural differences. + +In regard to the sterility of hybrids in successive generations; though +Gärtner was enabled to rear some hybrids, carefully guarding them from +a cross with either pure parent, for six or seven, and in one case for +ten generations, yet he asserts positively that their fertility never +increased, but generally greatly decreased. I do not doubt that this is +usually the case, and that the fertility often suddenly decreases in +the first few generations. Nevertheless I believe that in all these +experiments the fertility has been diminished by an independent cause, +namely, from close interbreeding. I have collected so large a body of +facts, showing +that close interbreeding lessens fertility, and, on the other hand, +that an occasional cross with a distinct individual or variety +increases fertility, that I cannot doubt the correctness of this almost +universal belief amongst breeders. Hybrids are seldom raised by +experimentalists in great numbers; and as the parent-species, or other +allied hybrids, generally grow in the same garden, the visits of +insects must be carefully prevented during the flowering season: hence +hybrids will generally be fertilised during each generation by their +own individual pollen; and I am convinced that this would be injurious +to their fertility, already lessened by their hybrid origin. I am +strengthened in this conviction by a remarkable statement repeatedly +made by Gärtner, namely, that if even the less fertile hybrids be +artificially fertilised with hybrid pollen of the same kind, their +fertility, notwithstanding the frequent ill effects of manipulation, +sometimes decidedly increases, and goes on increasing. Now, in +artificial fertilisation pollen is as often taken by chance (as I know +from my own experience) from the anthers of another flower, as from the +anthers of the flower itself which is to be fertilised; so that a cross +between two flowers, though probably on the same plant, would be thus +effected. Moreover, whenever complicated experiments are in progress, +so careful an observer as Gärtner would have castrated his hybrids, and +this would have insured in each generation a cross with the pollen from +a distinct flower, either from the same plant or from another plant of +the same hybrid nature. And thus, the strange fact of the increase of +fertility in the successive generations of _artificially fertilised_ +hybrids may, I believe, be accounted for by close interbreeding having +been avoided. + +Now let us turn to the results arrived at by the third most experienced +hybridiser, namely, the Honourable and +Reverend W. Herbert. He is as emphatic in his conclusion that some +hybrids are perfectly fertile—as fertile as the pure parent-species—as +are Kölreuter and Gärtner that some degree of sterility between +distinct species is a universal law of nature. He experimentised on +some of the very same species as did Gärtner. The difference in their +results may, I think, be in part accounted for by Herbert’s great +horticultural skill, and by his having hothouses at his command. Of his +many important statements I will here give only a single one as an +example, namely, that “every ovule in a pod of Crinum capense +fertilised by C. revolutum produced a plant, which (he says) I never +saw to occur in a case of its natural fecundation.” So that we here +have perfect, or even more than commonly perfect, fertility in a first +cross between two distinct species. + +This case of the Crinum leads me to refer to a most singular fact, +namely, that there are individual plants, as with certain species of +Lobelia, and with all the species of the genus Hippeastrum, which can +be far more easily fertilised by the pollen of another and distinct +species, than by their own pollen. For these plants have been found to +yield seed to the pollen of a distinct species, though quite sterile +with their own pollen, notwithstanding that their own pollen was found +to be perfectly good, for it fertilised distinct species. So that +certain individual plants and all the individuals of certain species +can actually be hybridised much more readily than they can be +self-fertilised! For instance, a bulb of Hippeastrum aulicum produced +four flowers; three were fertilised by Herbert with their own pollen, +and the fourth was subsequently fertilised by the pollen of a compound +hybrid descended from three other and distinct species: the result was +that “the ovaries of the three first flowers soon ceased to grow, and +after a +few days perished entirely, whereas the pod impregnated by the pollen +of the hybrid made vigorous growth and rapid progress to maturity, and +bore good seed, which vegetated freely.” In a letter to me, in 1839, +Mr. Herbert told me that he had then tried the experiment during five +years, and he continued to try it during several subsequent years, and +always with the same result. This result has, also, been confirmed by +other observers in the case of Hippeastrum with its sub-genera, and in +the case of some other genera, as Lobelia, Passiflora and Verbascum. +Although the plants in these experiments appeared perfectly healthy, +and although both the ovules and pollen of the same flower were +perfectly good with respect to other species, yet as they were +functionally imperfect in their mutual self-action, we must infer that +the plants were in an unnatural state. Nevertheless these facts show on +what slight and mysterious causes the lesser or greater fertility of +species when crossed, in comparison with the same species when +self-fertilised, sometimes depends. + +The practical experiments of horticulturists, though not made with +scientific precision, deserve some notice. It is notorious in how +complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, +Petunia, Rhododendron, etc., have been crossed, yet many of these +hybrids seed freely. For instance, Herbert asserts that a hybrid from +Calceolaria integrifolia and plantaginea, species most widely +dissimilar in general habit, “reproduced itself as perfectly as if it +had been a natural species from the mountains of Chile.” I have taken +some pains to ascertain the degree of fertility of some of the complex +crosses of Rhododendrons, and I am assured that many of them are +perfectly fertile. Mr. C. Noble, for instance, informs me that he +raises stocks for grafting from a hybrid +between Rhododendron Ponticum and Catawbiense, and that this hybrid +“seeds as freely as it is possible to imagine.” Had hybrids, when +fairly treated, gone on decreasing in fertility in each successive +generation, as Gärtner believes to be the case, the fact would have +been notorious to nurserymen. Horticulturists raise large beds of the +same hybrids, and such alone are fairly treated, for by insect agency +the several individuals of the same hybrid variety are allowed to +freely cross with each other, and the injurious influence of close +interbreeding is thus prevented. Any one may readily convince himself +of the efficiency of insect-agency by examining the flowers of the more +sterile kinds of hybrid rhododendrons, which produce no pollen, for he +will find on their stigmas plenty of pollen brought from other flowers. + +In regard to animals, much fewer experiments have been carefully tried +than with plants. If our systematic arrangements can be trusted, that +is if the genera of animals are as distinct from each other, as are the +genera of plants, then we may infer that animals more widely separated +in the scale of nature can be more easily crossed than in the case of +plants; but the hybrids themselves are, I think, more sterile. I doubt +whether any case of a perfectly fertile hybrid animal can be considered +as thoroughly well authenticated. It should, however, be borne in mind +that, owing to few animals breeding freely under confinement, few +experiments have been fairly tried: for instance, the canary-bird has +been crossed with nine other finches, but as not one of these nine +species breeds freely in confinement, we have no right to expect that +the first crosses between them and the canary, or that their hybrids, +should be perfectly fertile. Again, with respect to the fertility in +successive generations of the more fertile +hybrid animals, I hardly know of an instance in which two families of +the same hybrid have been raised at the same time from different +parents, so as to avoid the ill effects of close interbreeding. On the +contrary, brothers and sisters have usually been crossed in each +successive generation, in opposition to the constantly repeated +admonition of every breeder. And in this case, it is not at all +surprising that the inherent sterility in the hybrids should have gone +on increasing. If we were to act thus, and pair brothers and sisters in +the case of any pure animal, which from any cause had the least +tendency to sterility, the breed would assuredly be lost in a very few +generations. + +Although I do not know of any thoroughly well-authenticated cases of +perfectly fertile hybrid animals, I have some reason to believe that +the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus +colchicus with P. torquatus and with P. versicolor are perfectly +fertile. The hybrids from the common and Chinese geese (A. cygnoides), +species which are so different that they are generally ranked in +distinct genera, have often bred in this country with either pure +parent, and in one single instance they have bred _inter se_. This was +effected by Mr. Eyton, who raised two hybrids from the same parents but +from different hatches; and from these two birds he raised no less than +eight hybrids (grandchildren of the pure geese) from one nest. In +India, however, these cross-bred geese must be far more fertile; for I +am assured by two eminently capable judges, namely Mr. Blyth and Capt. +Hutton, that whole flocks of these crossed geese are kept in various +parts of the country; and as they are kept for profit, where neither +pure parent-species exists, they must certainly be highly fertile. + +A doctrine which originated with Pallas, has been +largely accepted by modern naturalists; namely, that most of our +domestic animals have descended from two or more aboriginal species, +since commingled by intercrossing. On this view, the aboriginal species +must either at first have produced quite fertile hybrids, or the +hybrids must have become in subsequent generations quite fertile under +domestication. This latter alternative seems to me the most probable, +and I am inclined to believe in its truth, although it rests on no +direct evidence. I believe, for instance, that our dogs have descended +from several wild stocks; yet, with perhaps the exception of certain +indigenous domestic dogs of South America, all are quite fertile +together; and analogy makes me greatly doubt, whether the several +aboriginal species would at first have freely bred together and have +produced quite fertile hybrids. So again there is reason to believe +that our European and the humped Indian cattle are quite fertile +together; but from facts communicated to me by Mr. Blyth, I think they +must be considered as distinct species. On this view of the origin of +many of our domestic animals, we must either give up the belief of the +almost universal sterility of distinct species of animals when crossed; +or we must look at sterility, not as an indelible characteristic, but +as one capable of being removed by domestication. + +Finally, looking to all the ascertained facts on the intercrossing of +plants and animals, it may be concluded that some degree of sterility, +both in first crosses and in hybrids,is an extremely general result; +but that it cannot, under our present state of knowledge, be considered +as absolutely universal. + +_Laws governing the Sterility of first Crosses and of Hybrids_.—We will +now consider a little more in detail the +circumstances and rules governing the sterility of first crosses and of +hybrids. Our chief object will be to see whether or not the rules +indicate that species have specially been endowed with this quality, in +order to prevent their crossing and blending together in utter +confusion. The following rules and conclusions are chiefly drawn up +from Gärtner’s admirable work on the hybridisation of plants. I have +taken much pains to ascertain how far the rules apply to animals, and +considering how scanty our knowledge is in regard to hybrid animals, I +have been surprised to find how generally the same rules apply to both +kingdoms. + +It has been already remarked, that the degree of fertility, both of +first crosses and of hybrids, graduates from zero to perfect fertility. +It is surprising in how many curious ways this gradation can be shown +to exist; but only the barest outline of the facts can here be given. +When pollen from a plant of one family is placed on the stigma of a +plant of a distinct family, it exerts no more influence than so much +inorganic dust. From this absolute zero of fertility, the pollen of +different species of the same genus applied to the stigma of some one +species, yields a perfect gradation in the number of seeds produced, up +to nearly complete or even quite complete fertility; and, as we have +seen, in certain abnormal cases, even to an excess of fertility, beyond +that which the plant’s own pollen will produce. So in hybrids +themselves, there are some which never have produced, and probably +never would produce, even with the pollen of either pure parent, a +single fertile seed: but in some of these cases a first trace of +fertility may be detected, by the pollen of one of the pure +parent-species causing the flower of the hybrid to wither earlier than +it otherwise would have done; and the early withering of the flower is +well known to be a sign +of incipient fertilisation. From this extreme degree of sterility we +have self-fertilised hybrids producing a greater and greater number of +seeds up to perfect fertility. + +Hybrids from two species which are very difficult to cross, and which +rarely produce any offspring, are generally very sterile; but the +parallelism between the difficulty of making a first cross, and the +sterility of the hybrids thus produced—two classes of facts which are +generally confounded together—is by no means strict. There are many +cases, in which two pure species can be united with unusual facility, +and produce numerous hybrid-offspring, yet these hybrids are remarkably +sterile. On the other hand, there are species which can be crossed very +rarely, or with extreme difficulty, but the hybrids, when at last +produced, are very fertile. Even within the limits of the same genus, +for instance in Dianthus, these two opposite cases occur. + +The fertility, both of first crosses and of hybrids, is more easily +affected by unfavourable conditions, than is the fertility of pure +species. But the degree of fertility is likewise innately variable; for +it is not always the same when the same two species are crossed under +the same circumstances, but depends in part upon the constitution of +the individuals which happen to have been chosen for the experiment. So +it is with hybrids, for their degree of fertility is often found to +differ greatly in the several individuals raised from seed out of the +same capsule and exposed to exactly the same conditions. + +By the term systematic affinity is meant, the resemblance between +species in structure and in constitution, more especially in the +structure of parts which are of high physiological importance and which +differ little in the allied species. Now the fertility of first crosses +between species, and of the hybrids produced from them, is largely +governed by their systematic affinity. This is clearly shown by hybrids +never having been raised between species ranked by systematists in +distinct families; and on the other hand, by very closely allied +species generally uniting with facility. But the correspondence between +systematic affinity and the facility of crossing is by no means strict. +A multitude of cases could be given of very closely allied species +which will not unite, or only with extreme difficulty; and on the other +hand of very distinct species which unite with the utmost facility. In +the same family there may be a genus, as Dianthus, in which very many +species can most readily be crossed; and another genus, as Silene, in +which the most persevering efforts have failed to produce between +extremely close species a single hybrid. Even within the limits of the +same genus, we meet with this same difference; for instance, the many +species of Nicotiana have been more largely crossed than the species of +almost any other genus; but Gärtner found that N. acuminata, which is +not a particularly distinct species, obstinately failed to fertilise, +or to be fertilised by, no less than eight other species of Nicotiana. +Very many analogous facts could be given. + +No one has been able to point out what kind, or what amount, of +difference in any recognisable character is sufficient to prevent two +species crossing. It can be shown that plants most widely different in +habit and general appearance, and having strongly marked differences in +every part of the flower, even in the pollen, in the fruit, and in the +cotyledons, can be crossed. Annual and perennial plants, deciduous and +evergreen trees, plants inhabiting different stations and fitted for +extremely different climates, can often be crossed with ease. + + +By a reciprocal cross between two species, I mean the case, for +instance, of a stallion-horse being first crossed with a female-ass, +and then a male-ass with a mare: these two species may then be said to +have been reciprocally crossed. There is often the widest possible +difference in the facility of making reciprocal crosses. Such cases are +highly important, for they prove that the capacity in any two species +to cross is often completely independent of their systematic affinity, +or of any recognisable difference in their whole organisation. On the +other hand, these cases clearly show that the capacity for crossing is +connected with constitutional differences imperceptible by us, and +confined to the reproductive system. This difference in the result of +reciprocal crosses between the same two species was long ago observed +by Kölreuter. To give an instance: Mirabilis jalappa can easily be +fertilised by the pollen of M. longiflora, and the hybrids thus +produced are sufficiently fertile; but Kölreuter tried more than two +hundred times, during eight following years, to fertilise reciprocally +M. longiflora with the pollen of M. jalappa, and utterly failed. +Several other equally striking cases could be given. Thuret has +observed the same fact with certain sea-weeds or Fuci. Gärtner, +moreover, found that this difference of facility in making reciprocal +crosses is extremely common in a lesser degree. He has observed it even +between forms so closely related (as Matthiola annua and glabra) that +many botanists rank them only as varieties. It is also a remarkable +fact, that hybrids raised from reciprocal crosses, though of course +compounded of the very same two species, the one species having first +been used as the father and then as the mother, generally differ in +fertility in a small, and occasionally in a high degree. + +Several other singular rules could be given from +Gärtner: for instance, some species have a remarkable power of crossing +with other species; other species of the same genus have a remarkable +power of impressing their likeness on their hybrid offspring; but these +two powers do not at all necessarily go together. There are certain +hybrids which instead of having, as is usual, an intermediate character +between their two parents, always closely resemble one of them; and +such hybrids, though externally so like one of their pure +parent-species, are with rare exceptions extremely sterile. So again +amongst hybrids which are usually intermediate in structure between +their parents, exceptional and abnormal individuals sometimes are born, +which closely resemble one of their pure parents; and these hybrids are +almost always utterly sterile, even when the other hybrids raised from +seed from the same capsule have a considerable degree of fertility. +These facts show how completely fertility in the hybrid is independent +of its external resemblance to either pure parent. + +Considering the several rules now given, which govern the fertility of +first crosses and of hybrids, we see that when forms, which must be +considered as good and distinct species, are united, their fertility +graduates from zero to perfect fertility, or even to fertility under +certain conditions in excess. That their fertility, besides being +eminently susceptible to favourable and unfavourable conditions, is +innately variable. That it is by no means always the same in degree in +the first cross and in the hybrids produced from this cross. That the +fertility of hybrids is not related to the degree in which they +resemble in external appearance either parent. And lastly, that the +facility of making a first cross between any two species is not always +governed by their systematic affinity or +degree of resemblance to each other. This latter statement is clearly +proved by reciprocal crosses between the same two species, for +according as the one species or the other is used as the father or the +mother, there is generally some difference, and occasionally the widest +possible difference, in the facility of effecting an union. The +hybrids, moreover, produced from reciprocal crosses often differ in +fertility. + +Now do these complex and singular rules indicate that species have been +endowed with sterility simply to prevent their becoming confounded in +nature? I think not. For why should the sterility be so extremely +different in degree, when various species are crossed, all of which we +must suppose it would be equally important to keep from blending +together? Why should the degree of sterility be innately variable in +the individuals of the same species? Why should some species cross with +facility, and yet produce very sterile hybrids; and other species cross +with extreme difficulty, and yet produce fairly fertile hybrids? Why +should there often be so great a difference in the result of a +reciprocal cross between the same two species? Why, it may even be +asked, has the production of hybrids been permitted? to grant to +species the special power of producing hybrids, and then to stop their +further propagation by different degrees of sterility, not strictly +related to the facility of the first union between their parents, seems +to be a strange arrangement. + +The foregoing rules and facts, on the other hand, appear to me clearly +to indicate that the sterility both of first crosses and of hybrids is +simply incidental or dependent on unknown differences, chiefly in the +reproductive systems, of the species which are crossed. The differences +being of so peculiar and limited a nature, +that, in reciprocal crosses between two species the male sexual element +of the one will often freely act on the female sexual element of the +other, but not in a reversed direction. It will be advisable to explain +a little more fully by an example what I mean by sterility being +incidental on other differences, and not a specially endowed quality. +As the capacity of one plant to be grafted or budded on another is so +entirely unimportant for its welfare in a state of nature, I presume +that no one will suppose that this capacity is a _specially_ endowed +quality, but will admit that it is incidental on differences in the +laws of growth of the two plants. We can sometimes see the reason why +one tree will not take on another, from differences in their rate of +growth, in the hardness of their wood, in the period of the flow or +nature of their sap, etc.; but in a multitude of cases we can assign no +reason whatever. Great diversity in the size of two plants, one being +woody and the other herbaceous, one being evergreen and the other +deciduous, and adaptation to widely different climates, does not always +prevent the two grafting together. As in hybridisation, so with +grafting, the capacity is limited by systematic affinity, for no one +has been able to graft trees together belonging to quite distinct +families; and, on the other hand, closely allied species, and varieties +of the same species, can usually, but not invariably, be grafted with +ease. But this capacity, as in hybridisation, is by no means absolutely +governed by systematic affinity. Although many distinct genera within +the same family have been grafted together, in other cases species of +the same genus will not take on each other. The pear can be grafted far +more readily on the quince, which is ranked as a distinct genus, than +on the apple, which is a member of the same genus. Even different +varieties of the pear take +with different degrees of facility on the quince; so do different +varieties of the apricot and peach on certain varieties of the plum. + +As Gärtner found that there was sometimes an innate difference in +different _individuals_ of the same two species in crossing; so Sagaret +believes this to be the case with different individuals of the same two +species in being grafted together. As in reciprocal crosses, the +facility of effecting an union is often very far from equal, so it +sometimes is in grafting; the common gooseberry, for instance, cannot +be grafted on the currant, whereas the currant will take, though with +difficulty, on the gooseberry. + +We have seen that the sterility of hybrids, which have their +reproductive organs in an imperfect condition, is a very different case +from the difficulty of uniting two pure species, which have their +reproductive organs perfect; yet these two distinct cases run to a +certain extent parallel. Something analogous occurs in grafting; for +Thouin found that three species of Robinia, which seeded freely on +their own roots, and which could be grafted with no great difficulty on +another species, when thus grafted were rendered barren. On the other +hand, certain species of Sorbus, when grafted on other species, yielded +twice as much fruit as when on their own roots. We are reminded by this +latter fact of the extraordinary case of Hippeastrum, Lobelia, etc., +which seeded much more freely when fertilised with the pollen of +distinct species, than when self-fertilised with their own pollen. + +We thus see, that although there is a clear and fundamental difference +between the mere adhesion of grafted stocks, and the union of the male +and female elements in the act of reproduction, yet that there is a +rude degree of parallelism in the results of grafting and +of crossing distinct species. And as we must look at the curious and +complex laws governing the facility with which trees can be grafted on +each other as incidental on unknown differences in their vegetative +systems, so I believe that the still more complex laws governing the +facility of first crosses, are incidental on unknown differences, +chiefly in their reproductive systems. These differences, in both +cases, follow to a certain extent, as might have been expected, +systematic affinity, by which every kind of resemblance and +dissimilarity between organic beings is attempted to be expressed. The +facts by no means seem to me to indicate that the greater or lesser +difficulty of either grafting or crossing together various species has +been a special endowment; although in the case of crossing, the +difficulty is as important for the endurance and stability of specific +forms, as in the case of grafting it is unimportant for their welfare. + +_Causes of the Sterility of first Crosses and of Hybrids_.—We may now +look a little closer at the probable causes of the sterility of first +crosses and of hybrids. These two cases are fundamentally different, +for, as just remarked, in the union of two pure species the male and +female sexual elements are perfect, whereas in hybrids they are +imperfect. Even in first crosses, the greater or lesser difficulty in +effecting a union apparently depends on several distinct causes. There +must sometimes be a physical impossibility in the male element reaching +the ovule, as would be the case with a plant having a pistil too long +for the pollen-tubes to reach the ovarium. It has also been observed +that when pollen of one species is placed on the stigma of a distantly +allied species, though the pollen-tubes protrude, they do not penetrate +the stigmatic surface. Again, the +male element may reach the female element, but be incapable of causing +an embryo to be developed, as seems to have been the case with some of +Thuret’s experiments on Fuci. No explanation can be given of these +facts, any more than why certain trees cannot be grafted on others. +Lastly, an embryo may be developed, and then perish at an early period. +This latter alternative has not been sufficiently attended to; but I +believe, from observations communicated to me by Mr. Hewitt, who has +had great experience in hybridising gallinaceous birds, that the early +death of the embryo is a very frequent cause of sterility in first +crosses. I was at first very unwilling to believe in this view; as +hybrids, when once born, are generally healthy and long-lived, as we +see in the case of the common mule. Hybrids, however, are differently +circumstanced before and after birth: when born and living in a country +where their two parents can live, they are generally placed under +suitable conditions of life. But a hybrid partakes of only half of the +nature and constitution of its mother, and therefore before birth, as +long as it is nourished within its mother’s womb or within the egg or +seed produced by the mother, it may be exposed to conditions in some +degree unsuitable, and consequently be liable to perish at an early +period; more especially as all very young beings seem eminently +sensitive to injurious or unnatural conditions of life. + +In regard to the sterility of hybrids, in which the sexual elements are +imperfectly developed, the case is very different. I have more than +once alluded to a large body of facts, which I have collected, showing +that when animals and plants are removed from their natural conditions, +they are extremely liable to have their reproductive systems seriously +affected. This, in fact, is +the great bar to the domestication of animals. Between the sterility +thus superinduced and that of hybrids, there are many points of +similarity. In both cases the sterility is independent of general +health, and is often accompanied by excess of size or great luxuriance. +In both cases, the sterility occurs in various degrees; in both, the +male element is the most liable to be affected; but sometimes the +female more than the male. In both, the tendency goes to a certain +extent with systematic affinity, for whole groups of animals and plants +are rendered impotent by the same unnatural conditions; and whole +groups of species tend to produce sterile hybrids. On the other hand, +one species in a group will sometimes resist great changes of +conditions with unimpaired fertility; and certain species in a group +will produce unusually fertile hybrids. No one can tell, till he tries, +whether any particular animal will breed under confinement or any plant +seed freely under culture; nor can he tell, till he tries, whether any +two species of a genus will produce more or less sterile hybrids. +Lastly, when organic beings are placed during several generations under +conditions not natural to them, they are extremely liable to vary, +which is due, as I believe, to their reproductive systems having been +specially affected, though in a lesser degree than when sterility +ensues. So it is with hybrids, for hybrids in successive generations +are eminently liable to vary, as every experimentalist has observed. + +Thus we see that when organic beings are placed under new and unnatural +conditions, and when hybrids are produced by the unnatural crossing of +two species, the reproductive system, independently of the general +state of health, is affected by sterility in a very similar manner. In +the one case, the conditions of life have been disturbed, though often +in so slight a degree as to +be inappreciable by us; in the other case, or that of hybrids, the +external conditions have remained the same, but the organisation has +been disturbed by two different structures and constitutions having +been blended into one. For it is scarcely possible that two +organisations should be compounded into one, without some disturbance +occurring in the development, or periodical action, or mutual relation +of the different parts and organs one to another, or to the conditions +of life. When hybrids are able to breed _inter se_, they transmit to +their offspring from generation to generation the same compounded +organisation, and hence we need not be surprised that their sterility, +though in some degree variable, rarely diminishes. + +It must, however, be confessed that we cannot understand, excepting on +vague hypotheses, several facts with respect to the sterility of +hybrids; for instance, the unequal fertility of hybrids produced from +reciprocal crosses; or the increased sterility in those hybrids which +occasionally and exceptionally resemble closely either pure parent. Nor +do I pretend that the foregoing remarks go to the root of the matter: +no explanation is offered why an organism, when placed under unnatural +conditions, is rendered sterile. All that I have attempted to show, is +that in two cases, in some respects allied, sterility is the common +result,—in the one case from the conditions of life having been +disturbed, in the other case from the organisation having been +disturbed by two organisations having been compounded into one. + +It may seem fanciful, but I suspect that a similar parallelism extends +to an allied yet very different class of facts. It is an old and almost +universal belief, founded, I think, on a considerable body of evidence, +that slight changes in the conditions of life are beneficial to all +living things. We see this acted on by +farmers and gardeners in their frequent exchanges of seed, tubers, +etc., from one soil or climate to another, and back again. During the +convalescence of animals, we plainly see that great benefit is derived +from almost any change in the habits of life. Again, both with plants +and animals, there is abundant evidence, that a cross between very +distinct individuals of the same species, that is between members of +different strains or sub-breeds, gives vigour and fertility to the +offspring. I believe, indeed, from the facts alluded to in our fourth +chapter, that a certain amount of crossing is indispensable even with +hermaphrodites; and that close interbreeding continued during several +generations between the nearest relations, especially if these be kept +under the same conditions of life, always induces weakness and +sterility in the progeny. + +Hence it seems that, on the one hand, slight changes in the conditions +of life benefit all organic beings, and on the other hand, that slight +crosses, that is crosses between the males and females of the same +species which have varied and become slightly different, give vigour +and fertility to the offspring. But we have seen that greater changes, +or changes of a particular nature, often render organic beings in some +degree sterile; and that greater crosses, that is crosses between males +and females which have become widely or specifically different, produce +hybrids which are generally sterile in some degree. I cannot persuade +myself that this parallelism is an accident or an illusion. Both series +of facts seem to be connected together by some common but unknown bond, +which is essentially related to the principle of life. + +_Fertility of Varieties when crossed, and of their Mongrel +offspring_.—It may be urged, as a most forcible argument, +that there must be some essential distinction between species and +varieties, and that there must be some error in all the foregoing +remarks, inasmuch as varieties, however much they may differ from each +other in external appearance, cross with perfect facility, and yield +perfectly fertile offspring. I fully admit that this is almost +invariably the case. But if we look to varieties produced under nature, +we are immediately involved in hopeless difficulties; for if two +hitherto reputed varieties be found in any degree sterile together, +they are at once ranked by most naturalists as species. For instance, +the blue and red pimpernel, the primrose and cowslip, which are +considered by many of our best botanists as varieties, are said by +Gärtner not to be quite fertile when crossed, and he consequently ranks +them as undoubted species. If we thus argue in a circle, the fertility +of all varieties produced under nature will assuredly have to be +granted. + +If we turn to varieties, produced, or supposed to have been produced, +under domestication, we are still involved in doubt. For when it is +stated, for instance, that the German Spitz dog unites more easily than +other dogs with foxes, or that certain South American indigenous +domestic dogs do not readily cross with European dogs, the explanation +which will occur to everyone, and probably the true one, is that these +dogs have descended from several aboriginally distinct species. +Nevertheless the perfect fertility of so many domestic varieties, +differing widely from each other in appearance, for instance of the +pigeon or of the cabbage, is a remarkable fact; more especially when we +reflect how many species there are, which, though resembling each other +most closely, are utterly sterile when intercrossed. Several +considerations, however, render the fertility of domestic varieties +less remarkable than +at first appears. It can, in the first place, be clearly shown that +mere external dissimilarity between two species does not determine +their greater or lesser degree of sterility when crossed; and we may +apply the same rule to domestic varieties. In the second place, some +eminent naturalists believe that a long course of domestication tends +to eliminate sterility in the successive generations of hybrids, which +were at first only slightly sterile; and if this be so, we surely ought +not to expect to find sterility both appearing and disappearing under +nearly the same conditions of life. Lastly, and this seems to me by far +the most important consideration, new races of animals and plants are +produced under domestication by man’s methodical and unconscious power +of selection, for his own use and pleasure: he neither wishes to +select, nor could select, slight differences in the reproductive +system, or other constitutional differences correlated with the +reproductive system. He supplies his several varieties with the same +food; treats them in nearly the same manner, and does not wish to alter +their general habits of life. Nature acts uniformly and slowly during +vast periods of time on the whole organisation, in any way which may be +for each creature’s own good; and thus she may, either directly, or +more probably indirectly, through correlation, modify the reproductive +system in the several descendants from any one species. Seeing this +difference in the process of selection, as carried on by man and +nature, we need not be surprised at some difference in the result. + +I have as yet spoken as if the varieties of the same species were +invariably fertile when intercrossed. But it seems to me impossible to +resist the evidence of the existence of a certain amount of sterility +in the few following cases, which I will briefly abstract. The evidence +is at least as good as that from which we believe +in the sterility of a multitude of species. The evidence is, also, +derived from hostile witnesses, who in all other cases consider +fertility and sterility as safe criterions of specific distinction. +Gärtner kept during several years a dwarf kind of maize with yellow +seeds, and a tall variety with red seeds, growing near each other in +his garden; and although these plants have separated sexes, they never +naturally crossed. He then fertilised thirteen flowers of the one with +the pollen of the other; but only a single head produced any seed, and +this one head produced only five grains. Manipulation in this case +could not have been injurious, as the plants have separated sexes. No +one, I believe, has suspected that these varieties of maize are +distinct species; and it is important to notice that the hybrid plants +thus raised were themselves _perfectly_ fertile; so that even Gärtner +did not venture to consider the two varieties as specifically distinct. + +Girou de Buzareingues crossed three varieties of gourd, which like the +maize has separated sexes, and he asserts that their mutual +fertilisation is by so much the less easy as their differences are +greater. How far these experiments may be trusted, I know not; but the +forms experimentised on, are ranked by Sagaret, who mainly founds his +classification by the test of infertility, as varieties. + +The following case is far more remarkable, and seems at first quite +incredible; but it is the result of an astonishing number of +experiments made during many years on nine species of Verbascum, by so +good an observer and so hostile a witness, as Gärtner: namely, that +yellow and white varieties of the same species of Verbascum when +intercrossed produce less seed, than do either coloured varieties when +fertilised with pollen from their own coloured flowers. Moreover, he +asserts that when +yellow and white varieties of one species are crossed with yellow and +white varieties of a _distinct_ species, more seed is produced by the +crosses between the same coloured flowers, than between those which are +differently coloured. Yet these varieties of Verbascum present no other +difference besides the mere colour of the flower; and one variety can +sometimes be raised from the seed of the other. + +From observations which I have made on certain varieties of hollyhock, +I am inclined to suspect that they present analogous facts. + +Kölreuter, whose accuracy has been confirmed by every subsequent +observer, has proved the remarkable fact, that one variety of the +common tobacco is more fertile, when crossed with a widely distinct +species, than are the other varieties. He experimentised on five forms, +which are commonly reputed to be varieties, and which he tested by the +severest trial, namely, by reciprocal crosses, and he found their +mongrel offspring perfectly fertile. But one of these five varieties, +when used either as father or mother, and crossed with the Nicotiana +glutinosa, always yielded hybrids not so sterile as those which were +produced from the four other varieties when crossed with N. glutinosa. +Hence the reproductive system of this one variety must have been in +some manner and in some degree modified. + +From these facts; from the great difficulty of ascertaining the +infertility of varieties in a state of nature, for a supposed variety +if infertile in any degree would generally be ranked as species; from +man selecting only external characters in the production of the most +distinct domestic varieties, and from not wishing or being able to +produce recondite and functional differences in the reproductive +system; from these several considerations and facts, I do not think +that the very general +fertility of varieties can be proved to be of universal occurrence, or +to form a fundamental distinction between varieties and species. The +general fertility of varieties does not seem to me sufficient to +overthrow the view which I have taken with respect to the very general, +but not invariable, sterility of first crosses and of hybrids, namely, +that it is not a special endowment, but is incidental on slowly +acquired modifications, more especially in the reproductive systems of +the forms which are crossed. + +_Hybrids and Mongrels compared, independently of their +fertility_.—Independently of the question of fertility, the offspring +of species when crossed and of varieties when crossed may be compared +in several other respects. Gärtner, whose strong wish was to draw a +marked line of distinction between species and varieties, could find +very few and, as it seems to me, quite unimportant differences between +the so-called hybrid offspring of species, and the so-called mongrel +offspring of varieties. And, on the other hand, they agree most closely +in very many important respects. + +I shall here discuss this subject with extreme brevity. The most +important distinction is, that in the first generation mongrels are +more variable than hybrids; but Gärtner admits that hybrids from +species which have long been cultivated are often variable in the first +generation; and I have myself seen striking instances of this fact. +Gärtner further admits that hybrids between very closely allied species +are more variable than those from very distinct species; and this shows +that the difference in the degree of variability graduates away. When +mongrels and the more fertile hybrids are propagated for several +generations an extreme amount of variability in their offspring is +notorious; +but some few cases both of hybrids and mongrels long retaining +uniformity of character could be given. The variability, however, in +the successive generations of mongrels is, perhaps, greater than in +hybrids. + +This greater variability of mongrels than of hybrids does not seem to +me at all surprising. For the parents of mongrels are varieties, and +mostly domestic varieties (very few experiments having been tried on +natural varieties), and this implies in most cases that there has been +recent variability; and therefore we might expect that such variability +would often continue and be super-added to that arising from the mere +act of crossing. The slight degree of variability in hybrids from the +first cross or in the first generation, in contrast with their extreme +variability in the succeeding generations, is a curious fact and +deserves attention. For it bears on and corroborates the view which I +have taken on the cause of ordinary variability; namely, that it is due +to the reproductive system being eminently sensitive to any change in +the conditions of life, being thus often rendered either impotent or at +least incapable of its proper function of producing offspring identical +with the parent-form. Now hybrids in the first generation are descended +from species (excluding those long cultivated) which have not had their +reproductive systems in any way affected, and they are not variable; +but hybrids themselves have their reproductive systems seriously +affected, and their descendants are highly variable. + +But to return to our comparison of mongrels and hybrids: Gärtner states +that mongrels are more liable than hybrids to revert to either +parent-form; but this, if it be true, is certainly only a difference in +degree. Gärtner further insists that when any two species, although +most closely allied to each other, are +crossed with a third species, the hybrids are widely different from +each other; whereas if two very distinct varieties of one species are +crossed with another species, the hybrids do not differ much. But this +conclusion, as far as I can make out, is founded on a single +experiment; and seems directly opposed to the results of several +experiments made by Kölreuter. + +These alone are the unimportant differences, which Gärtner is able to +point out, between hybrid and mongrel plants. On the other hand, the +resemblance in mongrels and in hybrids to their respective parents, +more especially in hybrids produced from nearly related species, +follows according to Gärtner the same laws. When two species are +crossed, one has sometimes a prepotent power of impressing its likeness +on the hybrid; and so I believe it to be with varieties of plants. With +animals one variety certainly often has this prepotent power over +another variety. Hybrid plants produced from a reciprocal cross, +generally resemble each other closely; and so it is with mongrels from +a reciprocal cross. Both hybrids and mongrels can be reduced to either +pure parent-form, by repeated crosses in successive generations with +either parent. + +These several remarks are apparently applicable to animals; but the +subject is here excessively complicated, partly owing to the existence +of secondary sexual characters; but more especially owing to prepotency +in transmitting likeness running more strongly in one sex than in the +other, both when one species is crossed with another, and when one +variety is crossed with another variety. For instance, I think those +authors are right, who maintain that the ass has a prepotent power over +the horse, so that both the mule and the hinny more resemble the ass +than the horse; but that the prepotency runs more strongly in the +male-ass than in +the female, so that the mule, which is the offspring of the male-ass +and mare, is more like an ass, than is the hinny, which is the +offspring of the female-ass and stallion. + +Much stress has been laid by some authors on the supposed fact, that +mongrel animals alone are born closely like one of their parents; but +it can be shown that this does sometimes occur with hybrids; yet I +grant much less frequently with hybrids than with mongrels. Looking to +the cases which I have collected of cross-bred animals closely +resembling one parent, the resemblances seem chiefly confined to +characters almost monstrous in their nature, and which have suddenly +appeared—such as albinism, melanism, deficiency of tail or horns, or +additional fingers and toes; and do not relate to characters which have +been slowly acquired by selection. Consequently, sudden reversions to +the perfect character of either parent would be more likely to occur +with mongrels, which are descended from varieties often suddenly +produced and semi-monstrous in character, than with hybrids, which are +descended from species slowly and naturally produced. On the whole I +entirely agree with Dr. Prosper Lucas, who, after arranging an enormous +body of facts with respect to animals, comes to the conclusion, that +the laws of resemblance of the child to its parents are the same, +whether the two parents differ much or little from each other, namely +in the union of individuals of the same variety, or of different +varieties, or of distinct species. + +Laying aside the question of fertility and sterility, in all other +respects there seems to be a general and close similarity in the +offspring of crossed species, and of crossed varieties. If we look at +species as having been specially created, and at varieties as having +been produced by secondary laws, this similarity would be an +astonishing fact. But it harmonises perfectly with the view that there +is no essential distinction between species and varieties. + +_Summary of Chapter_.—First crosses between forms sufficiently distinct +to be ranked as species, and their hybrids, are very generally, but not +universally, sterile. The sterility is of all degrees, and is often so +slight that the two most careful experimentalists who have ever lived, +have come to diametrically opposite conclusions in ranking forms by +this test. The sterility is innately variable in individuals of the +same species, and is eminently susceptible of favourable and +unfavourable conditions. The degree of sterility does not strictly +follow systematic affinity, but is governed by several curious and +complex laws. It is generally different, and sometimes widely +different, in reciprocal crosses between the same two species. It is +not always equal in degree in a first cross and in the hybrid produced +from this cross. + +In the same manner as in grafting trees, the capacity of one species or +variety to take on another, is incidental on generally unknown +differences in their vegetative systems, so in crossing, the greater or +less facility of one species to unite with another, is incidental on +unknown differences in their reproductive systems. There is no more +reason to think that species have been specially endowed with various +degrees of sterility to prevent them crossing and blending in nature, +than to think that trees have been specially endowed with various and +somewhat analogous degrees of difficulty in being grafted together in +order to prevent them becoming inarched in our forests. + +The sterility of first crosses between pure species, which have their +reproductive systems perfect, seems +to depend on several circumstances; in some cases largely on the early +death of the embryo. The sterility of hybrids, which have their +reproductive systems imperfect, and which have had this system and +their whole organisation disturbed by being compounded of two distinct +species, seems closely allied to that sterility which so frequently +affects pure species, when their natural conditions of life have been +disturbed. This view is supported by a parallelism of another +kind;—namely, that the crossing of forms only slightly different is +favourable to the vigour and fertility of their offspring; and that +slight changes in the conditions of life are apparently favourable to +the vigour and fertility of all organic beings. It is not surprising +that the degree of difficulty in uniting two species, and the degree of +sterility of their hybrid-offspring should generally correspond, though +due to distinct causes; for both depend on the amount of difference of +some kind between the species which are crossed. Nor is it surprising +that the facility of effecting a first cross, the fertility of the +hybrids produced, and the capacity of being grafted together—though +this latter capacity evidently depends on widely different +circumstances—should all run, to a certain extent, parallel with the +systematic affinity of the forms which are subjected to experiment; for +systematic affinity attempts to express all kinds of resemblance +between all species. + +First crosses between forms known to be varieties, or sufficiently +alike to be considered as varieties, and their mongrel offspring, are +very generally, but not quite universally, fertile. Nor is this nearly +general and perfect fertility surprising, when we remember how liable +we are to argue in a circle with respect to varieties in a state of +nature; and when we remember that the greater number of varieties have +been produced under domestication +by the selection of mere external differences, and not of differences +in the reproductive system. In all other respects, excluding fertility, +there is a close general resemblance between hybrids and mongrels. +Finally, then, the facts briefly given in this chapter do not seem to +me opposed to, but even rather to support the view, that there is no +fundamental distinction between species and varieties. + + + + +CHAPTER IX. +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + + +On the absence of intermediate varieties at the present day. On the +nature of extinct intermediate varieties; on their number. On the vast +lapse of time, as inferred from the rate of deposition and of +denudation. On the poorness of our palæontological collections. On the +intermittence of geological formations. On the absence of intermediate +varieties in any one formation. On the sudden appearance of groups of +species. On their sudden appearance in the lowest known fossiliferous +strata. + + +In the sixth chapter I enumerated the chief objections which might be +justly urged against the views maintained in this volume. Most of them +have now been discussed. One, namely the distinctness of specific +forms, and their not being blended together by innumerable transitional +links, is a very obvious difficulty. I assigned reasons why such links +do not commonly occur at the present day, under the circumstances +apparently most favourable for their presence, namely on an extensive +and continuous area with graduated physical conditions. I endeavoured +to show, that the life of each species depends in a more important +manner on the presence of other already defined organic forms, than on +climate; and, therefore, that the really governing conditions of life +do not graduate away quite insensibly like heat or moisture. I +endeavoured, also, to show that intermediate varieties, from existing +in lesser numbers than the forms which they connect, will generally be +beaten out and exterminated during the course of further modification +and improvement. The main cause, however, of innumerable intermediate +links not now occurring everywhere throughout nature depends +on the very process of natural selection, through which new varieties +continually take the places of and exterminate their parent-forms. But +just in proportion as this process of extermination has acted on an +enormous scale, so must the number of intermediate varieties, which +have formerly existed on the earth, be truly enormous. Why then is not +every geological formation and every stratum full of such intermediate +links? Geology assuredly does not reveal any such finely graduated +organic chain; and this, perhaps, is the most obvious and gravest +objection which can be urged against my theory. The explanation lies, +as I believe, in the extreme imperfection of the geological record. + +In the first place it should always be borne in mind what sort of +intermediate forms must, on my theory, have formerly existed. I have +found it difficult, when looking at any two species, to avoid picturing +to myself, forms _directly_ intermediate between them. But this is a +wholly false view; we should always look for forms intermediate between +each species and a common but unknown progenitor; and the progenitor +will generally have differed in some respects from all its modified +descendants. To give a simple illustration: the fantail and pouter +pigeons have both descended from the rock-pigeon; if we possessed all +the intermediate varieties which have ever existed, we should have an +extremely close series between both and the rock-pigeon; but we should +have no varieties directly intermediate between the fantail and pouter; +none, for instance, combining a tail somewhat expanded with a crop +somewhat enlarged, the characteristic features of these two breeds. +These two breeds, moreover, have become so much modified, that if we +had no historical or indirect evidence regarding their origin, it would +not have been possible to have +determined from a mere comparison of their structure with that of the +rock-pigeon, whether they had descended from this species or from some +other allied species, such as C. oenas. + +So with natural species, if we look to forms very distinct, for +instance to the horse and tapir, we have no reason to suppose that +links ever existed directly intermediate between them, but between each +and an unknown common parent. The common parent will have had in its +whole organisation much general resemblance to the tapir and to the +horse; but in some points of structure may have differed considerably +from both, even perhaps more than they differ from each other. Hence in +all such cases, we should be unable to recognise the parent-form of any +two or more species, even if we closely compared the structure of the +parent with that of its modified descendants, unless at the same time +we had a nearly perfect chain of the intermediate links. + +It is just possible by my theory, that one of two living forms might +have descended from the other; for instance, a horse from a tapir; and +in this case _direct_ intermediate links will have existed between +them. But such a case would imply that one form had remained for a very +long period unaltered, whilst its descendants had undergone a vast +amount of change; and the principle of competition between organism and +organism, between child and parent, will render this a very rare event; +for in all cases the new and improved forms of life will tend to +supplant the old and unimproved forms. + +By the theory of natural selection all living species have been +connected with the parent-species of each genus, by differences not +greater than we see between the varieties of the same species at the +present +day; and these parent-species, now generally extinct, have in their +turn been similarly connected with more ancient species; and so on +backwards, always converging to the common ancestor of each great +class. So that the number of intermediate and transitional links, +between all living and extinct species, must have been inconceivably +great. But assuredly, if this theory be true, such have lived upon this +earth. + +_On the lapse of Time_.—Independently of our not finding fossil remains +of such infinitely numerous connecting links, it may be objected, that +time will not have sufficed for so great an amount of organic change, +all changes having been effected very slowly through natural selection. +It is hardly possible for me even to recall to the reader, who may not +be a practical geologist, the facts leading the mind feebly to +comprehend the lapse of time. He who can read Sir Charles Lyell’s grand +work on the Principles of Geology, which the future historian will +recognise as having produced a revolution in natural science, yet does +not admit how incomprehensibly vast have been the past periods of time, +may at once close this volume. Not that it suffices to study the +Principles of Geology, or to read special treatises by different +observers on separate formations, and to mark how each author attempts +to give an inadequate idea of the duration of each formation or even +each stratum. A man must for years examine for himself great piles of +superimposed strata, and watch the sea at work grinding down old rocks +and making fresh sediment, before he can hope to comprehend anything of +the lapse of time, the monuments of which we see around us. + +It is good to wander along lines of sea-coast, when formed of +moderately hard rocks, and mark the +process of degradation. The tides in most cases reach the cliffs only +for a short time twice a day, and the waves eat into them only when +they are charged with sand or pebbles; for there is reason to believe +that pure water can effect little or nothing in wearing away rock. At +last the base of the cliff is undermined, huge fragments fall down, and +these remaining fixed, have to be worn away, atom by atom, until +reduced in size they can be rolled about by the waves, and then are +more quickly ground into pebbles, sand, or mud. But how often do we see +along the bases of retreating cliffs rounded boulders, all thickly +clothed by marine productions, showing how little they are abraded and +how seldom they are rolled about! Moreover, if we follow for a few +miles any line of rocky cliff, which is undergoing degradation, we find +that it is only here and there, along a short length or round a +promontory, that the cliffs are at the present time suffering. The +appearance of the surface and the vegetation show that elsewhere years +have elapsed since the waters washed their base. + +He who most closely studies the action of the sea on our shores, will, +I believe, be most deeply impressed with the slowness with which rocky +coasts are worn away. The observations on this head by Hugh Miller, and +by that excellent observer Mr. Smith of Jordan Hill, are most +impressive. With the mind thus impressed, let any one examine beds of +conglomerate many thousand feet in thickness, which, though probably +formed at a quicker rate than many other deposits, yet, from being +formed of worn and rounded pebbles, each of which bears the stamp of +time, are good to show how slowly the mass has been accumulated. Let +him remember Lyell’s profound remark, that the thickness and extent of +sedimentary formations +are the result and measure of the degradation which the earth’s crust +has elsewhere suffered. And what an amount of degradation is implied by +the sedimentary deposits of many countries! Professor Ramsay has given +me the maximum thickness, in most cases from actual measurement, in a +few cases from estimate, of each formation in different parts of Great +Britain; and this is the result:— + + Feet + Palæozoic strata (not including igneous beds)...57,154. Secondary + strata................................13,190. Tertiary + strata..................................2,240. + +—making altogether 72,584 feet; that is, very nearly thirteen and +three-quarters British miles. Some of these formations, which are +represented in England by thin beds, are thousands of feet in thickness +on the Continent. Moreover, between each successive formation, we have, +in the opinion of most geologists, enormously long blank periods. So +that the lofty pile of sedimentary rocks in Britain, gives but an +inadequate idea of the time which has elapsed during their +accumulation; yet what time this must have consumed! Good observers +have estimated that sediment is deposited by the great Mississippi +river at the rate of only 600 feet in a hundred thousand years. This +estimate may be quite erroneous; yet, considering over what wide spaces +very fine sediment is transported by the currents of the sea, the +process of accumulation in any one area must be extremely slow. + +But the amount of denudation which the strata have in many places +suffered, independently of the rate of accumulation of the degraded +matter, probably offers the best evidence of the lapse of time. I +remember having been much struck with the evidence of denudation, when +viewing volcanic islands, which have been +worn by the waves and pared all round into perpendicular cliffs of one +or two thousand feet in height; for the gentle slope of the +lava-streams, due to their formerly liquid state, showed at a glance +how far the hard, rocky beds had once extended into the open ocean. The +same story is still more plainly told by faults,—those great cracks +along which the strata have been upheaved on one side, or thrown down +on the other, to the height or depth of thousands of feet; for since +the crust cracked, the surface of the land has been so completely +planed down by the action of the sea, that no trace of these vast +dislocations is externally visible. + +The Craven fault, for instance, extends for upwards of 30 miles, and +along this line the vertical displacement of the strata has varied from +600 to 3000 feet. Professor Ramsay has published an account of a +downthrow in Anglesea of 2300 feet; and he informs me that he fully +believes there is one in Merionethshire of 12,000 feet; yet in these +cases there is nothing on the surface to show such prodigious +movements; the pile of rocks on the one or other side having been +smoothly swept away. The consideration of these facts impresses my mind +almost in the same manner as does the vain endeavour to grapple with +the idea of eternity. + +I am tempted to give one other case, the well-known one of the +denudation of the Weald. Though it must be admitted that the denudation +of the Weald has been a mere trifle, in comparison with that which has +removed masses of our palæozoic strata, in parts ten thousand feet in +thickness, as shown in Professor Ramsay’s masterly memoir on this +subject. Yet it is an admirable lesson to stand on the North Downs and +to look at the distant South Downs; for, remembering that at no great +distance to the west the northern and southern escarpments meet and +close, one can safely picture to +oneself the great dome of rocks which must have covered up the Weald +within so limited a period as since the latter part of the Chalk +formation. The distance from the northern to the southern Downs is +about 22 miles, and the thickness of the several formations is on an +average about 1100 feet, as I am informed by Professor Ramsay. But if, +as some geologists suppose, a range of older rocks underlies the Weald, +on the flanks of which the overlying sedimentary deposits might have +accumulated in thinner masses than elsewhere, the above estimate would +be erroneous; but this source of doubt probably would not greatly +affect the estimate as applied to the western extremity of the +district. If, then, we knew the rate at which the sea commonly wears +away a line of cliff of any given height, we could measure the time +requisite to have denuded the Weald. This, of course, cannot be done; +but we may, in order to form some crude notion on the subject, assume +that the sea would eat into cliffs 500 feet in height at the rate of +one inch in a century. This will at first appear much too small an +allowance; but it is the same as if we were to assume a cliff one yard +in height to be eaten back along a whole line of coast at the rate of +one yard in nearly every twenty-two years. I doubt whether any rock, +even as soft as chalk, would yield at this rate excepting on the most +exposed coasts; though no doubt the degradation of a lofty cliff would +be more rapid from the breakage of the fallen fragments. On the other +hand, I do not believe that any line of coast, ten or twenty miles in +length, ever suffers degradation at the same time along its whole +indented length; and we must remember that almost all strata contain +harder layers or nodules, which from long resisting attrition form a +breakwater at the base. Hence, under ordinary circumstances, I conclude +that for a cliff 500 feet in height, a denudation +of one inch per century for the whole length would be an ample +allowance. At this rate, on the above data, the denudation of the Weald +must have required 306,662,400 years; or say three hundred million +years. + +The action of fresh water on the gently inclined Wealden district, when +upraised, could hardly have been great, but it would somewhat reduce +the above estimate. On the other hand, during oscillations of level, +which we know this area has undergone, the surface may have existed for +millions of years as land, and thus have escaped the action of the sea: +when deeply submerged for perhaps equally long periods, it would, +likewise, have escaped the action of the coast-waves. So that in all +probability a far longer period than 300 million years has elapsed +since the latter part of the Secondary period. + +I have made these few remarks because it is highly important for us to +gain some notion, however imperfect, of the lapse of years. During each +of these years, over the whole world, the land and the water has been +peopled by hosts of living forms. What an infinite number of +generations, which the mind cannot grasp, must have succeeded each +other in the long roll of years! Now turn to our richest geological +museums, and what a paltry display we behold! + +_On the poorness of our Palæontological collections_.—That our +palæontological collections are very imperfect, is admitted by every +one. The remark of that admirable palæontologist, the late Edward +Forbes, should not be forgotten, namely, that numbers of our fossil +species are known and named from single and often broken specimens, or +from a few specimens collected on some one spot. Only a small portion +of the surface of the earth has been geologically explored, and no part +with +sufficient care, as the important discoveries made every year in Europe +prove. No organism wholly soft can be preserved. Shells and bones will +decay and disappear when left on the bottom of the sea, where sediment +is not accumulating. I believe we are continually taking a most +erroneous view, when we tacitly admit to ourselves that sediment is +being deposited over nearly the whole bed of the sea, at a rate +sufficiently quick to embed and preserve fossil remains. Throughout an +enormously large proportion of the ocean, the bright blue tint of the +water bespeaks its purity. The many cases on record of a formation +conformably covered, after an enormous interval of time, by another and +later formation, without the underlying bed having suffered in the +interval any wear and tear, seem explicable only on the view of the +bottom of the sea not rarely lying for ages in an unaltered condition. +The remains which do become embedded, if in sand or gravel, will when +the beds are upraised generally be dissolved by the percolation of +rain-water. I suspect that but few of the very many animals which live +on the beach between high and low watermark are preserved. For +instance, the several species of the Chthamalinæ (a sub-family of +sessile cirripedes) coat the rocks all over the world in infinite +numbers: they are all strictly littoral, with the exception of a single +Mediterranean species, which inhabits deep water and has been found +fossil in Sicily, whereas not one other species has hitherto been found +in any tertiary formation: yet it is now known that the genus +Chthamalus existed during the chalk period. The molluscan genus Chiton +offers a partially analogous case. + +With respect to the terrestrial productions which lived during the +Secondary and Palæozoic periods, it is superfluous to state that our +evidence from fossil +remains is fragmentary in an extreme degree. For instance, not a land +shell is known belonging to either of these vast periods, with one +exception discovered by Sir C. Lyell in the carboniferous strata of +North America. In regard to mammiferous remains, a single glance at the +historical table published in the Supplement to Lyell’s Manual, will +bring home the truth, how accidental and rare is their preservation, +far better than pages of detail. Nor is their rarity surprising, when +we remember how large a proportion of the bones of tertiary mammals +have been discovered either in caves or in lacustrine deposits; and +that not a cave or true lacustrine bed is known belonging to the age of +our secondary or palæozoic formations. + +But the imperfection in the geological record mainly results from +another and more important cause than any of the foregoing; namely, +from the several formations being separated from each other by wide +intervals of time. When we see the formations tabulated in written +works, or when we follow them in nature, it is difficult to avoid +believing that they are closely consecutive. But we know, for instance, +from Sir R. Murchison’s great work on Russia, what wide gaps there are +in that country between the superimposed formations; so it is in North +America, and in many other parts of the world. The most skilful +geologist, if his attention had been exclusively confined to these +large territories, would never have suspected that during the periods +which were blank and barren in his own country, great piles of +sediment, charged with new and peculiar forms of life, had elsewhere +been accumulated. And if in each separate territory, hardly any idea +can be formed of the length of time which has elapsed between the +consecutive formations, we may infer that this could nowhere be +ascertained. The frequent +and great changes in the mineralogical composition of consecutive +formations, generally implying great changes in the geography of the +surrounding lands, whence the sediment has been derived, accords with +the belief of vast intervals of time having elapsed between each +formation. + +But we can, I think, see why the geological formations of each region +are almost invariably intermittent; that is, have not followed each +other in close sequence. Scarcely any fact struck me more when +examining many hundred miles of the South American coasts, which have +been upraised several hundred feet within the recent period, than the +absence of any recent deposits sufficiently extensive to last for even +a short geological period. Along the whole west coast, which is +inhabited by a peculiar marine fauna, tertiary beds are so scantily +developed, that no record of several successive and peculiar marine +faunas will probably be preserved to a distant age. A little reflection +will explain why along the rising coast of the western side of South +America, no extensive formations with recent or tertiary remains can +anywhere be found, though the supply of sediment must for ages have +been great, from the enormous degradation of the coast-rocks and from +muddy streams entering the sea. The explanation, no doubt, is, that the +littoral and sub-littoral deposits are continually worn away, as soon +as they are brought up by the slow and gradual rising of the land +within the grinding action of the coast-waves. + +We may, I think, safely conclude that sediment must be accumulated in +extremely thick, solid, or extensive masses, in order to withstand the +incessant action of the waves, when first upraised and during +subsequent oscillations of level. Such thick and extensive +accumulations of sediment may be formed in two ways; either, +in profound depths of the sea, in which case, judging from the +researches of E. Forbes, we may conclude that the bottom will be +inhabited by extremely few animals, and the mass when upraised will +give a most imperfect record of the forms of life which then existed; +or, sediment may be accumulated to any thickness and extent over a +shallow bottom, if it continue slowly to subside. In this latter case, +as long as the rate of subsidence and supply of sediment nearly balance +each other, the sea will remain shallow and favourable for life, and +thus a fossiliferous formation thick enough, when upraised, to resist +any amount of degradation, may be formed. + +I am convinced that all our ancient formations, which are rich in +fossils, have thus been formed during subsidence. Since publishing my +views on this subject in 1845, I have watched the progress of Geology, +and have been surprised to note how author after author, in treating of +this or that great formation, has come to the conclusion that it was +accumulated during subsidence. I may add, that the only ancient +tertiary formation on the west coast of South America, which has been +bulky enough to resist such degradation as it has as yet suffered, but +which will hardly last to a distant geological age, was certainly +deposited during a downward oscillation of level, and thus gained +considerable thickness. + +All geological facts tell us plainly that each area has undergone +numerous slow oscillations of level, and apparently these oscillations +have affected wide spaces. Consequently formations rich in fossils and +sufficiently thick and extensive to resist subsequent degradation, may +have been formed over wide spaces during periods of subsidence, but +only where the supply of sediment was sufficient to keep the sea +shallow and to embed and +preserve the remains before they had time to decay. On the other hand, +as long as the bed of the sea remained stationary, _thick_ deposits +could not have been accumulated in the shallow parts, which are the +most favourable to life. Still less could this have happened during the +alternate periods of elevation; or, to speak more accurately, the beds +which were then accumulated will have been destroyed by being upraised +and brought within the limits of the coast-action. + +Thus the geological record will almost necessarily be rendered +intermittent. I feel much confidence in the truth of these views, for +they are in strict accordance with the general principles inculcated by +Sir C. Lyell; and E. Forbes independently arrived at a similar +conclusion. + +One remark is here worth a passing notice. During periods of elevation +the area of the land and of the adjoining shoal parts of the sea will +be increased, and new stations will often be formed;—all circumstances +most favourable, as previously explained, for the formation of new +varieties and species; but during such periods there will generally be +a blank in the geological record. On the other hand, during subsidence, +the inhabited area and number of inhabitants will decrease (excepting +the productions on the shores of a continent when first broken up into +an archipelago), and consequently during subsidence, though there will +be much extinction, fewer new varieties or species will be formed; and +it is during these very periods of subsidence, that our great deposits +rich in fossils have been accumulated. Nature may almost be said to +have guarded against the frequent discovery of her transitional or +linking forms. + +From the foregoing considerations it cannot be doubted that the +geological record, viewed as a whole, is extremely imperfect; but if we +confine our attention to any one formation, it becomes more difficult +to understand, +why we do not therein find closely graduated varieties between the +allied species which lived at its commencement and at its close. Some +cases are on record of the same species presenting distinct varieties +in the upper and lower parts of the same formation, but, as they are +rare, they may be here passed over. Although each formation has +indisputably required a vast number of years for its deposition, I can +see several reasons why each should not include a graduated series of +links between the species which then lived; but I can by no means +pretend to assign due proportional weight to the following +considerations. + +Although each formation may mark a very long lapse of years, each +perhaps is short compared with the period requisite to change one +species into another. I am aware that two palæontologists, whose +opinions are worthy of much deference, namely Bronn and Woodward, have +concluded that the average duration of each formation is twice or +thrice as long as the average duration of specific forms. But +insuperable difficulties, as it seems to me, prevent us coming to any +just conclusion on this head. When we see a species first appearing in +the middle of any formation, it would be rash in the extreme to infer +that it had not elsewhere previously existed. So again when we find a +species disappearing before the uppermost layers have been deposited, +it would be equally rash to suppose that it then became wholly extinct. +We forget how small the area of Europe is compared with the rest of the +world; nor have the several stages of the same formation throughout +Europe been correlated with perfect accuracy. + +With marine animals of all kinds, we may safely infer a large amount of +migration during climatal and other changes; and when we see a species +first appearing in any formation, the probability is that it +only then first immigrated into that area. It is well known, for +instance, that several species appeared somewhat earlier in the +palæozoic beds of North America than in those of Europe; time having +apparently been required for their migration from the American to the +European seas. In examining the latest deposits of various quarters of +the world, it has everywhere been noted, that some few still existing +species are common in the deposit, but have become extinct in the +immediately surrounding sea; or, conversely, that some are now abundant +in the neighbouring sea, but are rare or absent in this particular +deposit. It is an excellent lesson to reflect on the ascertained amount +of migration of the inhabitants of Europe during the Glacial period, +which forms only a part of one whole geological period; and likewise to +reflect on the great changes of level, on the inordinately great change +of climate, on the prodigious lapse of time, all included within this +same glacial period. Yet it may be doubted whether in any quarter of +the world, sedimentary deposits, _including fossil remains_, have gone +on accumulating within the same area during the whole of this period. +It is not, for instance, probable that sediment was deposited during +the whole of the glacial period near the mouth of the Mississippi, +within that limit of depth at which marine animals can flourish; for we +know what vast geographical changes occurred in other parts of America +during this space of time. When such beds as were deposited in shallow +water near the mouth of the Mississippi during some part of the glacial +period shall have been upraised, organic remains will probably first +appear and disappear at different levels, owing to the migration of +species and to geographical changes. And in the distant future, a +geologist examining these beds, might be tempted to conclude that the +average duration of life +of the embedded fossils had been less than that of the glacial period, +instead of having been really far greater, that is extending from +before the glacial epoch to the present day. + +In order to get a perfect gradation between two forms in the upper and +lower parts of the same formation, the deposit must have gone on +accumulating for a very long period, in order to have given sufficient +time for the slow process of variation; hence the deposit will +generally have to be a very thick one; and the species undergoing +modification will have had to live on the same area throughout this +whole time. But we have seen that a thick fossiliferous formation can +only be accumulated during a period of subsidence; and to keep the +depth approximately the same, which is necessary in order to enable the +same species to live on the same space, the supply of sediment must +nearly have counterbalanced the amount of subsidence. But this same +movement of subsidence will often tend to sink the area whence the +sediment is derived, and thus diminish the supply whilst the downward +movement continues. In fact, this nearly exact balancing between the +supply of sediment and the amount of subsidence is probably a rare +contingency; for it has been observed by more than one palæontologist, +that very thick deposits are usually barren of organic remains, except +near their upper or lower limits. + +It would seem that each separate formation, like the whole pile of +formations in any country, has generally been intermittent in its +accumulation. When we see, as is so often the case, a formation +composed of beds of different mineralogical composition, we may +reasonably suspect that the process of deposition has been much +interrupted, as a change in the currents of the sea and a supply of +sediment of a different nature will +generally have been due to geographical changes requiring much time. +Nor will the closest inspection of a formation give any idea of the +time which its deposition has consumed. Many instances could be given +of beds only a few feet in thickness, representing formations, +elsewhere thousands of feet in thickness, and which must have required +an enormous period for their accumulation; yet no one ignorant of this +fact would have suspected the vast lapse of time represented by the +thinner formation. Many cases could be given of the lower beds of a +formation having been upraised, denuded, submerged, and then re-covered +by the upper beds of the same formation,—facts, showing what wide, yet +easily overlooked, intervals have occurred in its accumulation. In +other cases we have the plainest evidence in great fossilised trees, +still standing upright as they grew, of many long intervals of time and +changes of level during the process of deposition, which would never +even have been suspected, had not the trees chanced to have been +preserved: thus, Messrs. Lyell and Dawson found carboniferous beds 1400 +feet thick in Nova Scotia, with ancient root-bearing strata, one above +the other, at no less than sixty-eight different levels. Hence, when +the same species occur at the bottom, middle, and top of a formation, +the probability is that they have not lived on the same spot during the +whole period of deposition, but have disappeared and reappeared, +perhaps many times, during the same geological period. So that if such +species were to undergo a considerable amount of modification during +any one geological period, a section would not probably include all the +fine intermediate gradations which must on my theory have existed +between them, but abrupt, though perhaps very slight, changes of form. + +It is all-important to remember that naturalists have +no golden rule by which to distinguish species and varieties; they +grant some little variability to each species, but when they meet with +a somewhat greater amount of difference between any two forms, they +rank both as species, unless they are enabled to connect them together +by close intermediate gradations. And this from the reasons just +assigned we can seldom hope to effect in any one geological section. +Supposing B and C to be two species, and a third, A, to be found in an +underlying bed; even if A were strictly intermediate between B and C, +it would simply be ranked as a third and distinct species, unless at +the same time it could be most closely connected with either one or +both forms by intermediate varieties. Nor should it be forgotten, as +before explained, that A might be the actual progenitor of B and C, and +yet might not at all necessarily be strictly intermediate between them +in all points of structure. So that we might obtain the parent-species +and its several modified descendants from the lower and upper beds of a +formation, and unless we obtained numerous transitional gradations, we +should not recognise their relationship, and should consequently be +compelled to rank them all as distinct species. + +It is notorious on what excessively slight differences many +palæontologists have founded their species; and they do this the more +readily if the specimens come from different sub-stages of the same +formation. Some experienced conchologists are now sinking many of the +very fine species of D’Orbigny and others into the rank of varieties; +and on this view we do find the kind of evidence of change which on my +theory we ought to find. Moreover, if we look to rather wider +intervals, namely, to distinct but consecutive stages of the same great +formation, we find that the embedded fossils, though almost universally +ranked as specifically different, +yet are far more closely allied to each other than are the species +found in more widely separated formations; but to this subject I shall +have to return in the following chapter. + +One other consideration is worth notice: with animals and plants that +can propagate rapidly and are not highly locomotive, there is reason to +suspect, as we have formerly seen, that their varieties are generally +at first local; and that such local varieties do not spread widely and +supplant their parent-forms until they have been modified and perfected +in some considerable degree. According to this view, the chance of +discovering in a formation in any one country all the early stages of +transition between any two forms, is small, for the successive changes +are supposed to have been local or confined to some one spot. Most +marine animals have a wide range; and we have seen that with plants it +is those which have the widest range, that oftenest present varieties; +so that with shells and other marine animals, it is probably those +which have had the widest range, far exceeding the limits of the known +geological formations of Europe, which have oftenest given rise, first +to local varieties and ultimately to new species; and this again would +greatly lessen the chance of our being able to trace the stages of +transition in any one geological formation. + +It should not be forgotten, that at the present day, with perfect +specimens for examination, two forms can seldom be connected by +intermediate varieties and thus proved to be the same species, until +many specimens have been collected from many places; and in the case of +fossil species this could rarely be effected by palæontologists. We +shall, perhaps, best perceive the improbability of our being enabled to +connect species by numerous, fine, intermediate, fossil links, by +asking +ourselves whether, for instance, geologists at some future period will +be able to prove, that our different breeds of cattle, sheep, horses, +and dogs have descended from a single stock or from several aboriginal +stocks; or, again, whether certain sea-shells inhabiting the shores of +North America, which are ranked by some conchologists as distinct +species from their European representatives, and by other conchologists +as only varieties, are really varieties or are, as it is called, +specifically distinct. This could be effected only by the future +geologist discovering in a fossil state numerous intermediate +gradations; and such success seems to me improbable in the highest +degree. + +Geological research, though it has added numerous species to existing +and extinct genera, and has made the intervals between some few groups +less wide than they otherwise would have been, yet has done scarcely +anything in breaking down the distinction between species, by +connecting them together by numerous, fine, intermediate varieties; and +this not having been effected, is probably the gravest and most obvious +of all the many objections which may be urged against my views. Hence +it will be worth while to sum up the foregoing remarks, under an +imaginary illustration. The Malay Archipelago is of about the size of +Europe from the North Cape to the Mediterranean, and from Britain to +Russia; and therefore equals all the geological formations which have +been examined with any accuracy, excepting those of the United States +of America. I fully agree with Mr. Godwin-Austen, that the present +condition of the Malay Archipelago, with its numerous large islands +separated by wide and shallow seas, probably represents the former +state of Europe, when most of our formations were accumulating. The +Malay Archipelago is one of the richest regions of the +whole world in organic beings; yet if all the species were to be +collected which have ever lived there, how imperfectly would they +represent the natural history of the world! + +But we have every reason to believe that the terrestrial productions of +the archipelago would be preserved in an excessively imperfect manner +in the formations which we suppose to be there accumulating. I suspect +that not many of the strictly littoral animals, or of those which lived +on naked submarine rocks, would be embedded; and those embedded in +gravel or sand, would not endure to a distant epoch. Wherever sediment +did not accumulate on the bed of the sea, or where it did not +accumulate at a sufficient rate to protect organic bodies from decay, +no remains could be preserved. + +In our archipelago, I believe that fossiliferous formations could be +formed of sufficient thickness to last to an age, as distant in +futurity as the secondary formations lie in the past, only during +periods of subsidence. These periods of subsidence would be separated +from each other by enormous intervals, during which the area would be +either stationary or rising; whilst rising, each fossiliferous +formation would be destroyed, almost as soon as accumulated, by the +incessant coast-action, as we now see on the shores of South America. +During the periods of subsidence there would probably be much +extinction of life; during the periods of elevation, there would be +much variation, but the geological record would then be least perfect. + +It may be doubted whether the duration of any one great period of +subsidence over the whole or part of the archipelago, together with a +contemporaneous accumulation of sediment, would _exceed_ the average +duration of the same specific forms; and these contingencies are +indispensable for the preservation of all the transitional gradations +between any two or more species. If such gradations were not fully +preserved, transitional varieties would merely appear as so many +distinct species. It is, also, probable that each great period of +subsidence would be interrupted by oscillations of level, and that +slight climatal changes would intervene during such lengthy periods; +and in these cases the inhabitants of the archipelago would have to +migrate, and no closely consecutive record of their modifications could +be preserved in any one formation. + +Very many of the marine inhabitants of the archipelago now range +thousands of miles beyond its confines; and analogy leads me to believe +that it would be chiefly these far-ranging species which would oftenest +produce new varieties; and the varieties would at first generally be +local or confined to one place, but if possessed of any decided +advantage, or when further modified and improved, they would slowly +spread and supplant their parent-forms. When such varieties returned to +their ancient homes, as they would differ from their former state, in a +nearly uniform, though perhaps extremely slight degree, they would, +according to the principles followed by many palæontologists, be ranked +as new and distinct species. + +If then, there be some degree of truth in these remarks, we have no +right to expect to find in our geological formations, an infinite +number of those fine transitional forms, which on my theory assuredly +have connected all the past and present species of the same group into +one long and branching chain of life. We ought only to look for a few +links, some more closely, some more distantly related to each other; +and these links, let them be ever so close, if found in different +stages of the same formation, would, by most palæontologists, +be ranked as distinct species. But I do not pretend that I should ever +have suspected how poor a record of the mutations of life, the best +preserved geological section presented, had not the difficulty of our +not discovering innumerable transitional links between the species +which appeared at the commencement and close of each formation, pressed +so hardly on my theory. + +_On the sudden appearance of whole groups of Allied Species_.—The +abrupt manner in which whole groups of species suddenly appear in +certain formations, has been urged by several palæontologists, for +instance, by Agassiz, Pictet, and by none more forcibly than by +Professor Sedgwick, as a fatal objection to the belief in the +transmutation of species. If numerous species, belonging to the same +genera or families, have really started into life all at once, the fact +would be fatal to the theory of descent with slow modification through +natural selection. For the development of a group of forms, all of +which have descended from some one progenitor, must have been an +extremely slow process; and the progenitors must have lived long ages +before their modified descendants. But we continually over-rate the +perfection of the geological record, and falsely infer, because certain +genera or families have not been found beneath a certain stage, that +they did not exist before that stage. We continually forget how large +the world is, compared with the area over which our geological +formations have been carefully examined; we forget that groups of +species may elsewhere have long existed and have slowly multiplied +before they invaded the ancient archipelagoes of Europe and of the +United States. We do not make due allowance for the enormous intervals +of time, which have +probably elapsed between our consecutive formations,—longer perhaps in +some cases than the time required for the accumulation of each +formation. These intervals will have given time for the multiplication +of species from some one or some few parent-forms; and in the +succeeding formation such species will appear as if suddenly created. + +I may here recall a remark formerly made, namely that it might require +a long succession of ages to adapt an organism to some new and peculiar +line of life, for instance to fly through the air; but that when this +had been effected, and a few species had thus acquired a great +advantage over other organisms, a comparatively short time would be +necessary to produce many divergent forms, which would be able to +spread rapidly and widely throughout the world. + +I will now give a few examples to illustrate these remarks; and to show +how liable we are to error in supposing that whole groups of species +have suddenly been produced. I may recall the well-known fact that in +geological treatises, published not many years ago, the great class of +mammals was always spoken of as having abruptly come in at the +commencement of the tertiary series. And now one of the richest known +accumulations of fossil mammals belongs to the middle of the secondary +series; and one true mammal has been discovered in the new red +sandstone at nearly the commencement of this great series. Cuvier used +to urge that no monkey occurred in any tertiary stratum; but now +extinct species have been discovered in India, South America, and in +Europe even as far back as the eocene stage. The most striking case, +however, is that of the Whale family; as these animals have huge bones, +are marine, and range over the world, the fact of not a single bone of +a whale having been discovered in +any secondary formation, seemed fully to justify the belief that this +great and distinct order had been suddenly produced in the interval +between the latest secondary and earliest tertiary formation. But now +we may read in the Supplement to Lyell’s ‘Manual,’ published in 1858, +clear evidence of the existence of whales in the upper greensand, some +time before the close of the secondary period. + +I may give another instance, which from having passed under my own eyes +has much struck me. In a memoir on Fossil Sessile Cirripedes, I have +stated that, from the number of existing and extinct tertiary species; +from the extraordinary abundance of the individuals of many species all +over the world, from the Arctic regions to the equator, inhabiting +various zones of depths from the upper tidal limits to 50 fathoms; from +the perfect manner in which specimens are preserved in the oldest +tertiary beds; from the ease with which even a fragment of a valve can +be recognised; from all these circumstances, I inferred that had +sessile cirripedes existed during the secondary periods, they would +certainly have been preserved and discovered; and as not one species +had been discovered in beds of this age, I concluded that this great +group had been suddenly developed at the commencement of the tertiary +series. This was a sore trouble to me, adding as I thought one more +instance of the abrupt appearance of a great group of species. But my +work had hardly been published, when a skilful palæontologist, M. +Bosquet, sent me a drawing of a perfect specimen of an unmistakeable +sessile cirripede, which he had himself extracted from the chalk of +Belgium. And, as if to make the case as striking as possible, this +sessile cirripede was a Chthamalus, a very common, large, and +ubiquitous genus, of which not one specimen has as yet been found even +in any tertiary +stratum. Hence we now positively know that sessile cirripedes existed +during the secondary period; and these cirripedes might have been the +progenitors of our many tertiary and existing species. + +The case most frequently insisted on by palæontologists of the +apparently sudden appearance of a whole group of species, is that of +the teleostean fishes, low down in the Chalk period. This group +includes the large majority of existing species. Lately, Professor +Pictet has carried their existence one sub-stage further back; and some +palæontologists believe that certain much older fishes, of which the +affinities are as yet imperfectly known, are really teleostean. +Assuming, however, that the whole of them did appear, as Agassiz +believes, at the commencement of the chalk formation, the fact would +certainly be highly remarkable; but I cannot see that it would be an +insuperable difficulty on my theory, unless it could likewise be shown +that the species of this group appeared suddenly and simultaneously +throughout the world at this same period. It is almost superfluous to +remark that hardly any fossil-fish are known from south of the equator; +and by running through Pictet’s Palæontology it will be seen that very +few species are known from several formations in Europe. Some few +families of fish now have a confined range; the teleostean fish might +formerly have had a similarly confined range, and after having been +largely developed in some one sea, might have spread widely. Nor have +we any right to suppose that the seas of the world have always been so +freely open from south to north as they are at present. Even at this +day, if the Malay Archipelago were converted into land, the tropical +parts of the Indian Ocean would form a large and perfectly enclosed +basin, in which any great group of marine animals might be multiplied; +and +here they would remain confined, until some of the species became +adapted to a cooler climate, and were enabled to double the southern +capes of Africa or Australia, and thus reach other and distant seas. + +From these and similar considerations, but chiefly from our ignorance +of the geology of other countries beyond the confines of Europe and the +United States; and from the revolution in our palæontological ideas on +many points, which the discoveries of even the last dozen years have +effected, it seems to me to be about as rash in us to dogmatize on the +succession of organic beings throughout the world, as it would be for a +naturalist to land for five minutes on some one barren point in +Australia, and then to discuss the number and range of its productions. + +_On the sudden appearance of groups of Allied Species in the lowest +known fossiliferous strata_.—There is another and allied difficulty, +which is much graver. I allude to the manner in which numbers of +species of the same group, suddenly appear in the lowest known +fossiliferous rocks. Most of the arguments which have convinced me that +all the existing species of the same group have descended from one +progenitor, apply with nearly equal force to the earliest known +species. For instance, I cannot doubt that all the Silurian trilobites +have descended from some one crustacean, which must have lived long +before the Silurian age, and which probably differed greatly from any +known animal. Some of the most ancient Silurian animals, as the +Nautilus, Lingula, etc., do not differ much from living species; and it +cannot on my theory be supposed, that these old species were the +progenitors of all the species of the orders to which they belong, for +they do not present characters in any degree intermediate between them. +If, moreover, they had been the progenitors of these orders, they would +almost certainly have been long ago supplanted and exterminated by +their numerous and improved descendants. + +Consequently, if my theory be true, it is indisputable that before the +lowest Silurian stratum was deposited, long periods elapsed, as long +as, or probably far longer than, the whole interval from the Silurian +age to the present day; and that during these vast, yet quite unknown, +periods of time, the world swarmed with living creatures. + +To the question why we do not find records of these vast primordial +periods, I can give no satisfactory answer. Several of the most eminent +geologists, with Sir R. Murchison at their head, are convinced that we +see in the organic remains of the lowest Silurian stratum the dawn of +life on this planet. Other highly competent judges, as Lyell and the +late E. Forbes, dispute this conclusion. We should not forget that only +a small portion of the world is known with accuracy. M. Barrande has +lately added another and lower stage to the Silurian system, abounding +with new and peculiar species. Traces of life have been detected in the +Longmynd beds beneath Barrande’s so-called primordial zone. The +presence of phosphatic nodules and bituminous matter in some of the +lowest azoic rocks, probably indicates the former existence of life at +these periods. But the difficulty of understanding the absence of vast +piles of fossiliferous strata, which on my theory no doubt were +somewhere accumulated before the Silurian epoch, is very great. If +these most ancient beds had been wholly worn away by denudation, or +obliterated by metamorphic action, we ought to find only small remnants +of the formations next succeeding them in age, and these ought to be +very generally in +a metamorphosed condition. But the descriptions which we now possess of +the Silurian deposits over immense territories in Russia and in North +America, do not support the view, that the older a formation is, the +more it has suffered the extremity of denudation and metamorphism. + +The case at present must remain inexplicable; and may be truly urged as +a valid argument against the views here entertained. To show that it +may hereafter receive some explanation, I will give the following +hypothesis. From the nature of the organic remains, which do not appear +to have inhabited profound depths, in the several formations of Europe +and of the United States; and from the amount of sediment, miles in +thickness, of which the formations are composed, we may infer that from +first to last large islands or tracts of land, whence the sediment was +derived, occurred in the neighbourhood of the existing continents of +Europe and North America. But we do not know what was the state of +things in the intervals between the successive formations; whether +Europe and the United States during these intervals existed as dry +land, or as a submarine surface near land, on which sediment was not +deposited, or again as the bed of an open and unfathomable sea. + +Looking to the existing oceans, which are thrice as extensive as the +land, we see them studded with many islands; but not one oceanic island +is as yet known to afford even a remnant of any palæozoic or secondary +formation. Hence we may perhaps infer, that during the palæozoic and +secondary periods, neither continents nor continental islands existed +where our oceans now extend; for had they existed there, palæozoic and +secondary formations would in all probability have been accumulated +from sediment derived from their wear and +tear; and would have been at least partially upheaved by the +oscillations of level, which we may fairly conclude must have +intervened during these enormously long periods. If then we may infer +anything from these facts, we may infer that where our oceans now +extend, oceans have extended from the remotest period of which we have +any record; and on the other hand, that where continents now exist, +large tracts of land have existed, subjected no doubt to great +oscillations of level, since the earliest silurian period. The coloured +map appended to my volume on Coral Reefs, led me to conclude that the +great oceans are still mainly areas of subsidence, the great +archipelagoes still areas of oscillations of level, and the continents +areas of elevation. But have we any right to assume that things have +thus remained from eternity? Our continents seem to have been formed by +a preponderance, during many oscillations of level, of the force of +elevation; but may not the areas of preponderant movement have changed +in the lapse of ages? At a period immeasurably antecedent to the +silurian epoch, continents may have existed where oceans are now spread +out; and clear and open oceans may have existed where our continents +now stand. Nor should we be justified in assuming that if, for +instance, the bed of the Pacific Ocean were now converted into a +continent, we should there find formations older than the silurian +strata, supposing such to have been formerly deposited; for it might +well happen that strata which had subsided some miles nearer to the +centre of the earth, and which had been pressed on by an enormous +weight of superincumbent water, might have undergone far more +metamorphic action than strata which have always remained nearer to the +surface. The immense areas in some parts of the world, for instance in +South America, of bare metamorphic rocks, which +must have been heated under great pressure, have always seemed to me to +require some special explanation; and we may perhaps believe that we +see in these large areas, the many formations long anterior to the +silurian epoch in a completely metamorphosed condition. + +The several difficulties here discussed, namely our not finding in the +successive formations infinitely numerous transitional links between +the many species which now exist or have existed; the sudden manner in +which whole groups of species appear in our European formations; the +almost entire absence, as at present known, of fossiliferous formations +beneath the Silurian strata, are all undoubtedly of the gravest nature. +We see this in the plainest manner by the fact that all the most +eminent palæontologists, namely Cuvier, Owen, Agassiz, Barrande, +Falconer, E. Forbes, etc., and all our greatest geologists, as Lyell, +Murchison, Sedgwick, etc., have unanimously, often vehemently, +maintained the immutability of species. But I have reason to believe +that one great authority, Sir Charles Lyell, from further reflexion +entertains grave doubts on this subject. I feel how rash it is to +differ from these great authorities, to whom, with others, we owe all +our knowledge. Those who think the natural geological record in any +degree perfect, and who do not attach much weight to the facts and +arguments of other kinds given in this volume, will undoubtedly at once +reject my theory. For my part, following out Lyell’s metaphor, I look +at the natural geological record, as a history of the world imperfectly +kept, and written in a changing dialect; of this history we possess the +last volume alone, relating only to two or three countries. Of this +volume, only here and there a short chapter has +been preserved; and of each page, only here and there a few lines. Each +word of the slowly-changing language, in which the history is supposed +to be written, being more or less different in the interrupted +succession of chapters, may represent the apparently abruptly changed +forms of life, entombed in our consecutive, but widely separated +formations. On this view, the difficulties above discussed are greatly +diminished, or even disappear. + + + + +CHAPTER X. +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + + +On the slow and successive appearance of new species. On their +different rates of change. Species once lost do not reappear. Groups of +species follow the same general rules in their appearance and +disappearance as do single species. On Extinction. On simultaneous +changes in the forms of life throughout the world. On the affinities of +extinct species to each other and to living species. On the state of +development of ancient forms. On the succession of the same types +within the same areas. Summary of preceding and present chapters. + + +Let us now see whether the several facts and rules relating to the +geological succession of organic beings, better accord with the common +view of the immutability of species, or with that of their slow and +gradual modification, through descent and natural selection. + +New species have appeared very slowly, one after another, both on the +land and in the waters. Lyell has shown that it is hardly possible to +resist the evidence on this head in the case of the several tertiary +stages; and every year tends to fill up the blanks between them, and to +make the percentage system of lost and new forms more gradual. In some +of the most recent beds, though undoubtedly of high antiquity if +measured by years, only one or two species are lost forms, and only one +or two are new forms, having here appeared for the first time, either +locally, or, as far as we know, on the face of the earth. If we may +trust the observations of Philippi in Sicily, the successive changes in +the marine inhabitants of that island have been many and most gradual. +The secondary formations are more broken; but, as Bronn has remarked, +neither the appearance +nor disappearance of their many now extinct species has been +simultaneous in each separate formation. + +Species of different genera and classes have not changed at the same +rate, or in the same degree. In the oldest tertiary beds a few living +shells may still be found in the midst of a multitude of extinct forms. +Falconer has given a striking instance of a similar fact, in an +existing crocodile associated with many strange and lost mammals and +reptiles in the sub-Himalayan deposits. The Silurian Lingula differs +but little from the living species of this genus; whereas most of the +other Silurian Molluscs and all the Crustaceans have changed greatly. +The productions of the land seem to change at a quicker rate than those +of the sea, of which a striking instance has lately been observed in +Switzerland. There is some reason to believe that organisms, considered +high in the scale of nature, change more quickly than those that are +low: though there are exceptions to this rule. The amount of organic +change, as Pictet has remarked, does not strictly correspond with the +succession of our geological formations; so that between each two +consecutive formations, the forms of life have seldom changed in +exactly the same degree. Yet if we compare any but the most closely +related formations, all the species will be found to have undergone +some change. When a species has once disappeared from the face of the +earth, we have reason to believe that the same identical form never +reappears. The strongest apparent exception to this latter rule, is +that of the so-called “colonies” of M. Barrande, which intrude for a +period in the midst of an older formation, and then allow the +pre-existing fauna to reappear; but Lyell’s explanation, namely, that +it is a case of temporary migration from a distinct geographical +province, seems to me satisfactory. + + +These several facts accord well with my theory. I believe in no fixed +law of development, causing all the inhabitants of a country to change +abruptly, or simultaneously, or to an equal degree. The process of +modification must be extremely slow. The variability of each species is +quite independent of that of all others. Whether such variability be +taken advantage of by natural selection, and whether the variations be +accumulated to a greater or lesser amount, thus causing a greater or +lesser amount of modification in the varying species, depends on many +complex contingencies,—on the variability being of a beneficial nature, +on the power of intercrossing, on the rate of breeding, on the slowly +changing physical conditions of the country, and more especially on the +nature of the other inhabitants with which the varying species comes +into competition. Hence it is by no means surprising that one species +should retain the same identical form much longer than others; or, if +changing, that it should change less. We see the same fact in +geographical distribution; for instance, in the land-shells and +coleopterous insects of Madeira having come to differ considerably from +their nearest allies on the continent of Europe, whereas the marine +shells and birds have remained unaltered. We can perhaps understand the +apparently quicker rate of change in terrestrial and in more highly +organised productions compared with marine and lower productions, by +the more complex relations of the higher beings to their organic and +inorganic conditions of life, as explained in a former chapter. When +many of the inhabitants of a country have become modified and improved, +we can understand, on the principle of competition, and on that of the +many all-important relations of organism to organism, that any form +which does not become in some degree modified and improved, +will be liable to be exterminated. Hence we can see why all the species +in the same region do at last, if we look to wide enough intervals of +time, become modified; for those which do not change will become +extinct. + +In members of the same class the average amount of change, during long +and equal periods of time, may, perhaps, be nearly the same; but as the +accumulation of long-enduring fossiliferous formations depends on great +masses of sediment having been deposited on areas whilst subsiding, our +formations have been almost necessarily accumulated at wide and +irregularly intermittent intervals; consequently the amount of organic +change exhibited by the fossils embedded in consecutive formations is +not equal. Each formation, on this view, does not mark a new and +complete act of creation, but only an occasional scene, taken almost at +hazard, in a slowly changing drama. + +We can clearly understand why a species when once lost should never +reappear, even if the very same conditions of life, organic and +inorganic, should recur. For though the offspring of one species might +be adapted (and no doubt this has occurred in innumerable instances) to +fill the exact place of another species in the economy of nature, and +thus supplant it; yet the two forms—the old and the new—would not be +identically the same; for both would almost certainly inherit different +characters from their distinct progenitors. For instance, it is just +possible, if our fantail-pigeons were all destroyed, that fanciers, by +striving during long ages for the same object, might make a new breed +hardly distinguishable from our present fantail; but if the parent +rock-pigeon were also destroyed, and in nature we have every reason to +believe that the parent-form will generally be supplanted and +exterminated by its improved offspring, it is quite incredible that a +fantail, identical with the existing breed, could be raised from any +other species of pigeon, or even from the other well-established races +of the domestic pigeon, for the newly-formed fantail would be almost +sure to inherit from its new progenitor some slight characteristic +differences. + +Groups of species, that is, genera and families, follow the same +general rules in their appearance and disappearance as do single +species, changing more or less quickly, and in a greater or lesser +degree. A group does not reappear after it has once disappeared; or its +existence, as long as it lasts, is continuous. I am aware that there +are some apparent exceptions to this rule, but the exceptions are +surprisingly few, so few, that E. Forbes, Pictet, and Woodward (though +all strongly opposed to such views as I maintain) admit its truth; and +the rule strictly accords with my theory. For as all the species of the +same group have descended from some one species, it is clear that as +long as any species of the group have appeared in the long succession +of ages, so long must its members have continuously existed, in order +to have generated either new and modified or the same old and +unmodified forms. Species of the genus Lingula, for instance, must have +continuously existed by an unbroken succession of generations, from the +lowest Silurian stratum to the present day. + +We have seen in the last chapter that the species of a group sometimes +falsely appear to have come in abruptly; and I have attempted to give +an explanation of this fact, which if true would have been fatal to my +views. But such cases are certainly exceptional; the general rule being +a gradual increase in number, till the group reaches its maximum, and +then, sooner or later, it gradually decreases. If the +number of the species of a genus, or the number of the genera of a +family, be represented by a vertical line of varying thickness, +crossing the successive geological formations in which the species are +found, the line will sometimes falsely appear to begin at its lower +end, not in a sharp point, but abruptly; it then gradually thickens +upwards, sometimes keeping for a space of equal thickness, and +ultimately thins out in the upper beds, marking the decrease and final +extinction of the species. This gradual increase in number of the +species of a group is strictly conformable with my theory; as the +species of the same genus, and the genera of the same family, can +increase only slowly and progressively; for the process of modification +and the production of a number of allied forms must be slow and +gradual,—one species giving rise first to two or three varieties, these +being slowly converted into species, which in their turn produce by +equally slow steps other species, and so on, like the branching of a +great tree from a single stem, till the group becomes large. + +_On Extinction_.—We have as yet spoken only incidentally of the +disappearance of species and of groups of species. On the theory of +natural selection the extinction of old forms and the production of new +and improved forms are intimately connected together. The old notion of +all the inhabitants of the earth having been swept away at successive +periods by catastrophes, is very generally given up, even by those +geologists, as Elie de Beaumont, Murchison, Barrande, etc., whose +general views would naturally lead them to this conclusion. On the +contrary, we have every reason to believe, from the study of the +tertiary formations, that species and groups of species gradually +disappear, one after another, first from one spot, then from another, +and +finally from the world. Both single species and whole groups of species +last for very unequal periods; some groups, as we have seen, having +endured from the earliest known dawn of life to the present day; some +having disappeared before the close of the palæozoic period. No fixed +law seems to determine the length of time during which any single +species or any single genus endures. There is reason to believe that +the complete extinction of the species of a group is generally a slower +process than their production: if the appearance and disappearance of a +group of species be represented, as before, by a vertical line of +varying thickness, the line is found to taper more gradually at its +upper end, which marks the progress of extermination, than at its lower +end, which marks the first appearance and increase in numbers of the +species. In some cases, however, the extermination of whole groups of +beings, as of ammonites towards the close of the secondary period, has +been wonderfully sudden. + +The whole subject of the extinction of species has been involved in the +most gratuitous mystery. Some authors have even supposed that as the +individual has a definite length of life, so have species a definite +duration. No one I think can have marvelled more at the extinction of +species, than I have done. When I found in La Plata the tooth of a +horse embedded with the remains of Mastodon, Megatherium, Toxodon, and +other extinct monsters, which all co-existed with still living shells +at a very late geological period, I was filled with astonishment; for +seeing that the horse, since its introduction by the Spaniards into +South America, has run wild over the whole country and has increased in +numbers at an unparalleled rate, I asked myself what could so recently +have exterminated the former horse under conditions of life apparently +so favourable. But +how utterly groundless was my astonishment! Professor Owen soon +perceived that the tooth, though so like that of the existing horse, +belonged to an extinct species. Had this horse been still living, but +in some degree rare, no naturalist would have felt the least surprise +at its rarity; for rarity is the attribute of a vast number of species +of all classes, in all countries. If we ask ourselves why this or that +species is rare, we answer that something is unfavourable in its +conditions of life; but what that something is, we can hardly ever +tell. On the supposition of the fossil horse still existing as a rare +species, we might have felt certain from the analogy of all other +mammals, even of the slow-breeding elephant, and from the history of +the naturalisation of the domestic horse in South America, that under +more favourable conditions it would in a very few years have stocked +the whole continent. But we could not have told what the unfavourable +conditions were which checked its increase, whether some one or several +contingencies, and at what period of the horse’s life, and in what +degree, they severally acted. If the conditions had gone on, however +slowly, becoming less and less favourable, we assuredly should not have +perceived the fact, yet the fossil horse would certainly have become +rarer and rarer, and finally extinct;—its place being seized on by some +more successful competitor. + +It is most difficult always to remember that the increase of every +living being is constantly being checked by unperceived injurious +agencies; and that these same unperceived agencies are amply sufficient +to cause rarity, and finally extinction. We see in many cases in the +more recent tertiary formations, that rarity precedes extinction; and +we know that this has been the progress of events with those animals +which have +been exterminated, either locally or wholly, through man’s agency. I +may repeat what I published in 1845, namely, that to admit that species +generally become rare before they become extinct—to feel no surprise at +the rarity of a species, and yet to marvel greatly when it ceases to +exist, is much the same as to admit that sickness in the individual is +the forerunner of death—to feel no surprise at sickness, but when the +sick man dies, to wonder and to suspect that he died by some unknown +deed of violence. + +The theory of natural selection is grounded on the belief that each new +variety, and ultimately each new species, is produced and maintained by +having some advantage over those with which it comes into competition; +and the consequent extinction of less-favoured forms almost inevitably +follows. It is the same with our domestic productions: when a new and +slightly improved variety has been raised, it at first supplants the +less improved varieties in the same neighbourhood; when much improved +it is transported far and near, like our short-horn cattle, and takes +the place of other breeds in other countries. Thus the appearance of +new forms and the disappearance of old forms, both natural and +artificial, are bound together. In certain flourishing groups, the +number of new specific forms which have been produced within a given +time is probably greater than that of the old forms which have been +exterminated; but we know that the number of species has not gone on +indefinitely increasing, at least during the later geological periods, +so that looking to later times we may believe that the production of +new forms has caused the extinction of about the same number of old +forms. + +The competition will generally be most severe, as formerly explained +and illustrated by examples, between the forms which are most like each +other in all respects. +Hence the improved and modified descendants of a species will generally +cause the extermination of the parent-species; and if many new forms +have been developed from any one species, the nearest allies of that +species, _i.e._ the species of the same genus, will be the most liable +to extermination. Thus, as I believe, a number of new species descended +from one species, that is a new genus, comes to supplant an old genus, +belonging to the same family. But it must often have happened that a +new species belonging to some one group will have seized on the place +occupied by a species belonging to a distinct group, and thus caused +its extermination; and if many allied forms be developed from the +successful intruder, many will have to yield their places; and it will +generally be allied forms, which will suffer from some inherited +inferiority in common. But whether it be species belonging to the same +or to a distinct class, which yield their places to other species which +have been modified and improved, a few of the sufferers may often long +be preserved, from being fitted to some peculiar line of life, or from +inhabiting some distant and isolated station, where they have escaped +severe competition. For instance, a single species of Trigonia, a great +genus of shells in the secondary formations, survives in the Australian +seas; and a few members of the great and almost extinct group of Ganoid +fishes still inhabit our fresh waters. Therefore the utter extinction +of a group is generally, as we have seen, a slower process than its +production. + +With respect to the apparently sudden extermination of whole families +or orders, as of Trilobites at the close of the palæozoic period and of +Ammonites at the close of the secondary period, we must remember what +has been already said on the probable wide intervals of time +between our consecutive formations; and in these intervals there may +have been much slow extermination. Moreover, when by sudden immigration +or by unusually rapid development, many species of a new group have +taken possession of a new area, they will have exterminated in a +correspondingly rapid manner many of the old inhabitants; and the forms +which thus yield their places will commonly be allied, for they will +partake of some inferiority in common. + +Thus, as it seems to me, the manner in which single species and whole +groups of species become extinct, accords well with the theory of +natural selection. We need not marvel at extinction; if we must marvel, +let it be at our presumption in imagining for a moment that we +understand the many complex contingencies, on which the existence of +each species depends. If we forget for an instant, that each species +tends to increase inordinately, and that some check is always in +action, yet seldom perceived by us, the whole economy of nature will be +utterly obscured. Whenever we can precisely say why this species is +more abundant in individuals than that; why this species and not +another can be naturalised in a given country; then, and not till then, +we may justly feel surprise why we cannot account for the extinction of +this particular species or group of species. + +_On the Forms of Life changing almost simultaneously throughout the +World_.—Scarcely any palæontological discovery is more striking than +the fact, that the forms of life change almost simultaneously +throughout the world. Thus our European Chalk formation can be +recognised in many distant parts of the world, under the most different +climates, where not a fragment of the mineral chalk itself can be +found; namely, in North +America, in equatorial South America, in Tierra del Fuego, at the Cape +of Good Hope, and in the peninsula of India. For at these distant +points, the organic remains in certain beds present an unmistakeable +degree of resemblance to those of the Chalk. It is not that the same +species are met with; for in some cases not one species is identically +the same, but they belong to the same families, genera, and sections of +genera, and sometimes are similarly characterised in such trifling +points as mere superficial sculpture. Moreover other forms, which are +not found in the Chalk of Europe, but which occur in the formations +either above or below, are similarly absent at these distant points of +the world. In the several successive palæozoic formations of Russia, +Western Europe and North America, a similar parallelism in the forms of +life has been observed by several authors: so it is, according to +Lyell, with the several European and North American tertiary deposits. +Even if the few fossil species which are common to the Old and New +Worlds be kept wholly out of view, the general parallelism in the +successive forms of life, in the stages of the widely separated +palæozoic and tertiary periods, would still be manifest, and the +several formations could be easily correlated. + +These observations, however, relate to the marine inhabitants of +distant parts of the world: we have not sufficient data to judge +whether the productions of the land and of fresh water change at +distant points in the same parallel manner. We may doubt whether they +have thus changed: if the Megatherium, Mylodon, Macrauchenia, and +Toxodon had been brought to Europe from La Plata, without any +information in regard to their geological position, no one would have +suspected that they had coexisted with still living sea-shells; but as +these anomalous monsters coexisted with the Mastodon +and Horse, it might at least have been inferred that they had lived +during one of the latter tertiary stages. + +When the marine forms of life are spoken of as having changed +simultaneously throughout the world, it must not be supposed that this +expression relates to the same thousandth or hundred-thousandth year, +or even that it has a very strict geological sense; for if all the +marine animals which live at the present day in Europe, and all those +that lived in Europe during the pleistocene period (an enormously +remote period as measured by years, including the whole glacial epoch), +were to be compared with those now living in South America or in +Australia, the most skilful naturalist would hardly be able to say +whether the existing or the pleistocene inhabitants of Europe resembled +most closely those of the southern hemisphere. So, again, several +highly competent observers believe that the existing productions of the +United States are more closely related to those which lived in Europe +during certain later tertiary stages, than to those which now live +here; and if this be so, it is evident that fossiliferous beds +deposited at the present day on the shores of North America would +hereafter be liable to be classed with somewhat older European beds. +Nevertheless, looking to a remotely future epoch, there can, I think, +be little doubt that all the more modern _marine_ formations, namely, +the upper pliocene, the pleistocene and strictly modern beds, of +Europe, North and South America, and Australia, from containing fossil +remains in some degree allied, and from not including those forms which +are only found in the older underlying deposits, would be correctly +ranked as simultaneous in a geological sense. + +The fact of the forms of life changing simultaneously, in the above +large sense, at distant parts of the world, has greatly struck those +admirable observers, MM. +de Verneuil and d’Archiac. After referring to the parallelism of the +palæozoic forms of life in various parts of Europe, they add, “If +struck by this strange sequence, we turn our attention to North +America, and there discover a series of analogous phenomena, it will +appear certain that all these modifications of species, their +extinction, and the introduction of new ones, cannot be owing to mere +changes in marine currents or other causes more or less local and +temporary, but depend on general laws which govern the whole animal +kingdom.” M. Barrande has made forcible remarks to precisely the same +effect. It is, indeed, quite futile to look to changes of currents, +climate, or other physical conditions, as the cause of these great +mutations in the forms of life throughout the world, under the most +different climates. We must, as Barrande has remarked, look to some +special law. We shall see this more clearly when we treat of the +present distribution of organic beings, and find how slight is the +relation between the physical conditions of various countries, and the +nature of their inhabitants. + +This great fact of the parallel succession of the forms of life +throughout the world, is explicable on the theory of natural selection. +New species are formed by new varieties arising, which have some +advantage over older forms; and those forms, which are already +dominant, or have some advantage over the other forms in their own +country, would naturally oftenest give rise to new varieties or +incipient species; for these latter must be victorious in a still +higher degree in order to be preserved and to survive. We have distinct +evidence on this head, in the plants which are dominant, that is, which +are commonest in their own homes, and are most widely diffused, having +produced the greatest number of new varieties. It is also natural that +the dominant, +varying, and far-spreading species, which already have invaded to a +certain extent the territories of other species, should be those which +would have the best chance of spreading still further, and of giving +rise in new countries to new varieties and species. The process of +diffusion may often be very slow, being dependent on climatal and +geographical changes, or on strange accidents, but in the long run the +dominant forms will generally succeed in spreading. The diffusion +would, it is probable, be slower with the terrestrial inhabitants of +distinct continents than with the marine inhabitants of the continuous +sea. We might therefore expect to find, as we apparently do find, a +less strict degree of parallel succession in the productions of the +land than of the sea. + +Dominant species spreading from any region might encounter still more +dominant species, and then their triumphant course, or even their +existence, would cease. We know not at all precisely what are all the +conditions most favourable for the multiplication of new and dominant +species; but we can, I think, clearly see that a number of individuals, +from giving a better chance of the appearance of favourable variations, +and that severe competition with many already existing forms, would be +highly favourable, as would be the power of spreading into new +territories. A certain amount of isolation, recurring at long intervals +of time, would probably be also favourable, as before explained. One +quarter of the world may have been most favourable for the production +of new and dominant species on the land, and another for those in the +waters of the sea. If two great regions had been for a long period +favourably circumstanced in an equal degree, whenever their inhabitants +met, the battle would be prolonged and severe; and some from one +birthplace and some from the other might be victorious. But in the +course of time, the +forms dominant in the highest degree, wherever produced, would tend +everywhere to prevail. As they prevailed, they would cause the +extinction of other and inferior forms; and as these inferior forms +would be allied in groups by inheritance, whole groups would tend +slowly to disappear; though here and there a single member might long +be enabled to survive. + +Thus, as it seems to me, the parallel, and, taken in a large sense, +simultaneous, succession of the same forms of life throughout the +world, accords well with the principle of new species having been +formed by dominant species spreading widely and varying; the new +species thus produced being themselves dominant owing to inheritance, +and to having already had some advantage over their parents or over +other species; these again spreading, varying, and producing new +species. The forms which are beaten and which yield their places to the +new and victorious forms, will generally be allied in groups, from +inheriting some inferiority in common; and therefore as new and +improved groups spread throughout the world, old groups will disappear +from the world; and the succession of forms in both ways will +everywhere tend to correspond. + +There is one other remark connected with this subject worth making. I +have given my reasons for believing that all our greater fossiliferous +formations were deposited during periods of subsidence; and that blank +intervals of vast duration occurred during the periods when the bed of +the sea was either stationary or rising, and likewise when sediment was +not thrown down quickly enough to embed and preserve organic remains. +During these long and blank intervals I suppose that the inhabitants of +each region underwent a considerable amount of modification and +extinction, and that there was much migration from +other parts of the world. As we have reason to believe that large areas +are affected by the same movement, it is probable that strictly +contemporaneous formations have often been accumulated over very wide +spaces in the same quarter of the world; but we are far from having any +right to conclude that this has invariably been the case, and that +large areas have invariably been affected by the same movements. When +two formations have been deposited in two regions during nearly, but +not exactly the same period, we should find in both, from the causes +explained in the foregoing paragraphs, the same general succession in +the forms of life; but the species would not exactly correspond; for +there will have been a little more time in the one region than in the +other for modification, extinction, and immigration. + +I suspect that cases of this nature have occurred in Europe. Mr. +Prestwich, in his admirable Memoirs on the eocene deposits of England +and France, is able to draw a close general parallelism between the +successive stages in the two countries; but when he compares certain +stages in England with those in France, although he finds in both a +curious accordance in the numbers of the species belonging to the same +genera, yet the species themselves differ in a manner very difficult to +account for, considering the proximity of the two areas,—unless, +indeed, it be assumed that an isthmus separated two seas inhabited by +distinct, but contemporaneous, faunas. Lyell has made similar +observations on some of the later tertiary formations. Barrande, also, +shows that there is a striking general parallelism in the successive +Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a +surprising amount of difference in the species. If the several +formations in these regions have not been deposited during the same +exact +periods,—a formation in one region often corresponding with a blank +interval in the other,—and if in both regions the species have gone on +slowly changing during the accumulation of the several formations and +during the long intervals of time between them; in this case, the +several formations in the two regions could be arranged in the same +order, in accordance with the general succession of the form of life, +and the order would falsely appear to be strictly parallel; +nevertheless the species would not all be the same in the apparently +corresponding stages in the two regions. + +_On the Affinities of extinct Species to each other, and to living +forms_.—Let us now look to the mutual affinities of extinct and living +species. They all fall into one grand natural system; and this fact is +at once explained on the principle of descent. The more ancient any +form is, the more, as a general rule, it differs from living forms. +But, as Buckland long ago remarked, all fossils can be classed either +in still existing groups, or between them. That the extinct forms of +life help to fill up the wide intervals between existing genera, +families, and orders, cannot be disputed. For if we confine our +attention either to the living or to the extinct alone, the series is +far less perfect than if we combine both into one general system. With +respect to the Vertebrata, whole pages could be filled with striking +illustrations from our great palæontologist, Owen, showing how extinct +animals fall in between existing groups. Cuvier ranked the Ruminants +and Pachyderms, as the two most distinct orders of mammals; but Owen +has discovered so many fossil links, that he has had to alter the whole +classification of these two orders; and has placed certain pachyderms +in the same sub-order with ruminants: for example, he dissolves by fine +gradations the apparently +wide difference between the pig and the camel. In regard to the +Invertebrata, Barrande, and a higher authority could not be named, +asserts that he is every day taught that palæozoic animals, though +belonging to the same orders, families, or genera with those living at +the present day, were not at this early epoch limited in such distinct +groups as they now are. + +Some writers have objected to any extinct species or group of species +being considered as intermediate between living species or groups. If +by this term it is meant that an extinct form is directly intermediate +in all its characters between two living forms, the objection is +probably valid. But I apprehend that in a perfectly natural +classification many fossil species would have to stand between living +species, and some extinct genera between living genera, even between +genera belonging to distinct families. The most common case, especially +with respect to very distinct groups, such as fish and reptiles, seems +to be, that supposing them to be distinguished at the present day from +each other by a dozen characters, the ancient members of the same two +groups would be distinguished by a somewhat lesser number of +characters, so that the two groups, though formerly quite distinct, at +that period made some small approach to each other. + +It is a common belief that the more ancient a form is, by so much the +more it tends to connect by some of its characters groups now widely +separated from each other. This remark no doubt must be restricted to +those groups which have undergone much change in the course of +geological ages; and it would be difficult to prove the truth of the +proposition, for every now and then even a living animal, as the +Lepidosiren, is discovered having affinities directed towards very +distinct groups. Yet if we compare the older Reptiles and +Batrachians, the older Fish, the older Cephalopods, and the eocene +Mammals, with the more recent members of the same classes, we must +admit that there is some truth in the remark. + +Let us see how far these several facts and inferences accord with the +theory of descent with modification. As the subject is somewhat +complex, I must request the reader to turn to the diagram in the fourth +chapter. We may suppose that the numbered letters represent genera, and +the dotted lines diverging from them the species in each genus. The +diagram is much too simple, too few genera and too few species being +given, but this is unimportant for us. The horizontal lines may +represent successive geological formations, and all the forms beneath +the uppermost line may be considered as extinct. The three existing +genera, _a_14, _q_14, _p_14, will form a small family; _b_14 and _f_14 +a closely allied family or sub-family; and _o_14, _e_14, _m_14, a third +family. These three families, together with the many extinct genera on +the several lines of descent diverging from the parent-form A, will +form an order; for all will have inherited something in common from +their ancient and common progenitor. On the principle of the continued +tendency to divergence of character, which was formerly illustrated by +this diagram, the more recent any form is, the more it will generally +differ from its ancient progenitor. Hence we can understand the rule +that the most ancient fossils differ most from existing forms. We must +not, however, assume that divergence of character is a necessary +contingency; it depends solely on the descendants from a species being +thus enabled to seize on many and different places in the economy of +nature. Therefore it is quite possible, as we have seen in the case of +some Silurian forms, that a species might go on being slightly +modified in relation to its slightly altered conditions of life, and +yet retain throughout a vast period the same general characteristics. +This is represented in the diagram by the letter F14. + +All the many forms, extinct and recent, descended from A, make, as +before remarked, one order; and this order, from the continued effects +of extinction and divergence of character, has become divided into +several sub-families and families, some of which are supposed to have +perished at different periods, and some to have endured to the present +day. + +By looking at the diagram we can see that if many of the extinct forms, +supposed to be embedded in the successive formations, were discovered +at several points low down in the series, the three existing families +on the uppermost line would be rendered less distinct from each other. +If, for instance, the genera _a_1, _a_5, _a_10, _f_8, _m_3, _m_6, _m_9 +were disinterred, these three families would be so closely linked +together that they probably would have to be united into one great +family, in nearly the same manner as has occurred with ruminants and +pachyderms. Yet he who objected to call the extinct genera, which thus +linked the living genera of three families together, intermediate in +character, would be justified, as they are intermediate, not directly, +but only by a long and circuitous course through many widely different +forms. If many extinct forms were to be discovered above one of the +middle horizontal lines or geological formations—for instance, above +Number VI.—but none from beneath this line, then only the two families +on the left hand (namely, _a_14, etc., and _b_14, etc.) would have to +be united into one family; and the two other families (namely, _a_14 to +_f_14 now including five genera, and _o_14 to _m_14) would yet remain +distinct. These two families, however, would be less distinct from each +other than they were before the +discovery of the fossils. If, for instance, we suppose the existing +genera of the two families to differ from each other by a dozen +characters, in this case the genera, at the early period marked VI., +would differ by a lesser number of characters; for at this early stage +of descent they have not diverged in character from the common +progenitor of the order, nearly so much as they subsequently diverged. +Thus it comes that ancient and extinct genera are often in some slight +degree intermediate in character between their modified descendants, or +between their collateral relations. + +In nature the case will be far more complicated than is represented in +the diagram; for the groups will have been more numerous, they will +have endured for extremely unequal lengths of time, and will have been +modified in various degrees. As we possess only the last volume of the +geological record, and that in a very broken condition, we have no +right to expect, except in very rare cases, to fill up wide intervals +in the natural system, and thus unite distinct families or orders. All +that we have a right to expect, is that those groups, which have within +known geological periods undergone much modification, should in the +older formations make some slight approach to each other; so that the +older members should differ less from each other in some of their +characters than do the existing members of the same groups; and this by +the concurrent evidence of our best palæontologists seems frequently to +be the case. + +Thus, on the theory of descent with modification, the main facts with +respect to the mutual affinities of the extinct forms of life to each +other and to living forms, seem to me explained in a satisfactory +manner. And they are wholly inexplicable on any other view. + +On this same theory, it is evident that the fauna of any great period +in the earth’s history will be intermediate +in general character between that which preceded and that which +succeeded it. Thus, the species which lived at the sixth great stage of +descent in the diagram are the modified offspring of those which lived +at the fifth stage, and are the parents of those which became still +more modified at the seventh stage; hence they could hardly fail to be +nearly intermediate in character between the forms of life above and +below. We must, however, allow for the entire extinction of some +preceding forms, and for the coming in of quite new forms by +immigration, and for a large amount of modification, during the long +and blank intervals between the successive formations. Subject to these +allowances, the fauna of each geological period undoubtedly is +intermediate in character, between the preceding and succeeding faunas. +I need give only one instance, namely, the manner in which the fossils +of the Devonian system, when this system was first discovered, were at +once recognised by palæontologists as intermediate in character between +those of the overlying carboniferous, and underlying Silurian system. +But each fauna is not necessarily exactly intermediate, as unequal +intervals of time have elapsed between consecutive formations. + +It is no real objection to the truth of the statement, that the fauna +of each period as a whole is nearly intermediate in character between +the preceding and succeeding faunas, that certain genera offer +exceptions to the rule. For instance, mastodons and elephants, when +arranged by Dr. Falconer in two series, first according to their mutual +affinities and then according to their periods of existence, do not +accord in arrangement. The species extreme in character are not the +oldest, or the most recent; nor are those which are intermediate in +character, intermediate in age. But +supposing for an instant, in this and other such cases, that the record +of the first appearance and disappearance of the species was perfect, +we have no reason to believe that forms successively produced +necessarily endure for corresponding lengths of time: a very ancient +form might occasionally last much longer than a form elsewhere +subsequently produced, especially in the case of terrestrial +productions inhabiting separated districts. To compare small things +with great: if the principal living and extinct races of the domestic +pigeon were arranged as well as they could be in serial affinity, this +arrangement would not closely accord with the order in time of their +production, and still less with the order of their disappearance; for +the parent rock-pigeon now lives; and many varieties between the +rock-pigeon and the carrier have become extinct; and carriers which are +extreme in the important character of length of beak originated earlier +than short-beaked tumblers, which are at the opposite end of the series +in this same respect. + +Closely connected with the statement, that the organic remains from an +intermediate formation are in some degree intermediate in character, is +the fact, insisted on by all palæontologists, that fossils from two +consecutive formations are far more closely related to each other, than +are the fossils from two remote formations. Pictet gives as a +well-known instance, the general resemblance of the organic remains +from the several stages of the chalk formation, though the species are +distinct in each stage. This fact alone, from its generality, seems to +have shaken Professor Pictet in his firm belief in the immutability of +species. He who is acquainted with the distribution of existing species +over the globe, will not attempt to account for the close resemblance +of the distinct species in closely consecutive +formations, by the physical conditions of the ancient areas having +remained nearly the same. Let it be remembered that the forms of life, +at least those inhabiting the sea, have changed almost simultaneously +throughout the world, and therefore under the most different climates +and conditions. Consider the prodigious vicissitudes of climate during +the pleistocene period, which includes the whole glacial period, and +note how little the specific forms of the inhabitants of the sea have +been affected. + +On the theory of descent, the full meaning of the fact of fossil +remains from closely consecutive formations, though ranked as distinct +species, being closely related, is obvious. As the accumulation of each +formation has often been interrupted, and as long blank intervals have +intervened between successive formations, we ought not to expect to +find, as I attempted to show in the last chapter, in any one or two +formations all the intermediate varieties between the species which +appeared at the commencement and close of these periods; but we ought +to find after intervals, very long as measured by years, but only +moderately long as measured geologically, closely allied forms, or, as +they have been called by some authors, representative species; and +these we assuredly do find. We find, in short, such evidence of the +slow and scarcely sensible mutation of specific forms, as we have a +just right to expect to find. + +_On the state of Development of Ancient Forms_.—There has been much +discussion whether recent forms are more highly developed than ancient. +I will not here enter on this subject, for naturalists have not as yet +defined to each other’s satisfaction what is meant by high and low +forms. But in one particular sense the +more recent forms must, on my theory, be higher than the more ancient; +for each new species is formed by having had some advantage in the +struggle for life over other and preceding forms. If under a nearly +similar climate, the eocene inhabitants of one quarter of the world +were put into competition with the existing inhabitants of the same or +some other quarter, the eocene fauna or flora would certainly be beaten +and exterminated; as would a secondary fauna by an eocene, and a +palæozoic fauna by a secondary fauna. I do not doubt that this process +of improvement has affected in a marked and sensible manner the +organisation of the more recent and victorious forms of life, in +comparison with the ancient and beaten forms; but I can see no way of +testing this sort of progress. Crustaceans, for instance, not the +highest in their own class, may have beaten the highest molluscs. From +the extraordinary manner in which European productions have recently +spread over New Zealand, and have seized on places which must have been +previously occupied, we may believe, if all the animals and plants of +Great Britain were set free in New Zealand, that in the course of time +a multitude of British forms would become thoroughly naturalized there, +and would exterminate many of the natives. On the other hand, from what +we see now occurring in New Zealand, and from hardly a single +inhabitant of the southern hemisphere having become wild in any part of +Europe, we may doubt, if all the productions of New Zealand were set +free in Great Britain, whether any considerable number would be enabled +to seize on places now occupied by our native plants and animals. Under +this point of view, the productions of Great Britain may be said to be +higher than those of New Zealand. Yet the most skilful naturalist from +an examination of the species +of the two countries could not have foreseen this result. + +Agassiz insists that ancient animals resemble to a certain extent the +embryos of recent animals of the same classes; or that the geological +succession of extinct forms is in some degree parallel to the +embryological development of recent forms. I must follow Pictet and +Huxley in thinking that the truth of this doctrine is very far from +proved. Yet I fully expect to see it hereafter confirmed, at least in +regard to subordinate groups, which have branched off from each other +within comparatively recent times. For this doctrine of Agassiz accords +well with the theory of natural selection. In a future chapter I shall +attempt to show that the adult differs from its embryo, owing to +variations supervening at a not early age, and being inherited at a +corresponding age. This process, whilst it leaves the embryo almost +unaltered, continually adds, in the course of successive generations, +more and more difference to the adult. + +Thus the embryo comes to be left as a sort of picture, preserved by +nature, of the ancient and less modified condition of each animal. This +view may be true, and yet it may never be capable of full proof. +Seeing, for instance, that the oldest known mammals, reptiles, and fish +strictly belong to their own proper classes, though some of these old +forms are in a slight degree less distinct from each other than are the +typical members of the same groups at the present day, it would be vain +to look for animals having the common embryological character of the +Vertebrata, until beds far beneath the lowest Silurian strata are +discovered—a discovery of which the chance is very small. + +_On the Succession of the same Types within the same +areas, during the later tertiary periods_.—Mr. Clift many years ago +showed that the fossil mammals from the Australian caves were closely +allied to the living marsupials of that continent. In South America, a +similar relationship is manifest, even to an uneducated eye, in the +gigantic pieces of armour like those of the armadillo, found in several +parts of La Plata; and Professor Owen has shown in the most striking +manner that most of the fossil mammals, buried there in such numbers, +are related to South American types. This relationship is even more +clearly seen in the wonderful collection of fossil bones made by MM. +Lund and Clausen in the caves of Brazil. I was so much impressed with +these facts that I strongly insisted, in 1839 and 1845, on this “law of +the succession of types,”—on “this wonderful relationship in the same +continent between the dead and the living.” Professor Owen has +subsequently extended the same generalisation to the mammals of the Old +World. We see the same law in this author’s restorations of the extinct +and gigantic birds of New Zealand. We see it also in the birds of the +caves of Brazil. Mr. Woodward has shown that the same law holds good +with sea-shells, but from the wide distribution of most genera of +molluscs, it is not well displayed by them. Other cases could be added, +as the relation between the extinct and living land-shells of Madeira; +and between the extinct and living brackish-water shells of the +Aralo-Caspian Sea. + +Now what does this remarkable law of the succession of the same types +within the same areas mean? He would be a bold man, who after comparing +the present climate of Australia and of parts of South America under +the same latitude, would attempt to account, on the one hand, by +dissimilar physical conditions for the dissimilarity of the inhabitants +of these two continents, +and, on the other hand, by similarity of conditions, for the uniformity +of the same types in each during the later tertiary periods. Nor can it +be pretended that it is an immutable law that marsupials should have +been chiefly or solely produced in Australia; or that Edentata and +other American types should have been solely produced in South America. +For we know that Europe in ancient times was peopled by numerous +marsupials; and I have shown in the publications above alluded to, that +in America the law of distribution of terrestrial mammals was formerly +different from what it now is. North America formerly partook strongly +of the present character of the southern half of the continent; and the +southern half was formerly more closely allied, than it is at present, +to the northern half. In a similar manner we know from Falconer and +Cautley’s discoveries, that northern India was formerly more closely +related in its mammals to Africa than it is at the present time. +Analogous facts could be given in relation to the distribution of +marine animals. + +On the theory of descent with modification, the great law of the long +enduring, but not immutable, succession of the same types within the +same areas, is at once explained; for the inhabitants of each quarter +of the world will obviously tend to leave in that quarter, during the +next succeeding period of time, closely allied though in some degree +modified descendants. If the inhabitants of one continent formerly +differed greatly from those of another continent, so will their +modified descendants still differ in nearly the same manner and degree. +But after very long intervals of time and after great geographical +changes, permitting much inter-migration, the feebler will yield to the +more dominant forms, and there will be nothing immutable in the laws of +past and present distribution. + + +It may be asked in ridicule, whether I suppose that the megatherium and +other allied huge monsters have left behind them in South America the +sloth, armadillo, and anteater, as their degenerate descendants. This +cannot for an instant be admitted. These huge animals have become +wholly extinct, and have left no progeny. But in the caves of Brazil, +there are many extinct species which are closely allied in size and in +other characters to the species still living in South America; and some +of these fossils may be the actual progenitors of living species. It +must not be forgotten that, on my theory, all the species of the same +genus have descended from some one species; so that if six genera, each +having eight species, be found in one geological formation, and in the +next succeeding formation there be six other allied or representative +genera with the same number of species, then we may conclude that only +one species of each of the six older genera has left modified +descendants, constituting the six new genera. The other seven species +of the old genera have all died out and have left no progeny. Or, which +would probably be a far commoner case, two or three species of two or +three alone of the six older genera will have been the parents of the +six new genera; the other old species and the other whole genera having +become utterly extinct. In failing orders, with the genera and species +decreasing in numbers, as apparently is the case of the Edentata of +South America, still fewer genera and species will have left modified +blood-descendants. + +_Summary of the preceding and present Chapters_.—I have attempted to +show that the geological record is extremely imperfect; that only a +small portion of the globe has been geologically explored with care; +that only +certain classes of organic beings have been largely preserved in a +fossil state; that the number both of specimens and of species, +preserved in our museums, is absolutely as nothing compared with the +incalculable number of generations which must have passed away even +during a single formation; that, owing to subsidence being necessary +for the accumulation of fossiliferous deposits thick enough to resist +future degradation, enormous intervals of time have elapsed between the +successive formations; that there has probably been more extinction +during the periods of subsidence, and more variation during the periods +of elevation, and during the latter the record will have been least +perfectly kept; that each single formation has not been continuously +deposited; that the duration of each formation is, perhaps, short +compared with the average duration of specific forms; that migration +has played an important part in the first appearance of new forms in +any one area and formation; that widely ranging species are those which +have varied most, and have oftenest given rise to new species; and that +varieties have at first often been local. All these causes taken +conjointly, must have tended to make the geological record extremely +imperfect, and will to a large extent explain why we do not find +interminable varieties, connecting together all the extinct and +existing forms of life by the finest graduated steps. + +He who rejects these views on the nature of the geological record, will +rightly reject my whole theory. For he may ask in vain where are the +numberless transitional links which must formerly have connected the +closely allied or representative species, found in the several stages +of the same great formation. He may disbelieve in the enormous +intervals of time which have elapsed between our consecutive +formations; he +may overlook how important a part migration must have played, when the +formations of any one great region alone, as that of Europe, are +considered; he may urge the apparent, but often falsely apparent, +sudden coming in of whole groups of species. He may ask where are the +remains of those infinitely numerous organisms which must have existed +long before the first bed of the Silurian system was deposited: I can +answer this latter question only hypothetically, by saying that as far +as we can see, where our oceans now extend they have for an enormous +period extended, and where our oscillating continents now stand they +have stood ever since the Silurian epoch; but that long before that +period, the world may have presented a wholly different aspect; and +that the older continents, formed of formations older than any known to +us, may now all be in a metamorphosed condition, or may lie buried +under the ocean. + +Passing from these difficulties, all the other great leading facts in +palæontology seem to me simply to follow on the theory of descent with +modification through natural selection. We can thus understand how it +is that new species come in slowly and successively; how species of +different classes do not necessarily change together, or at the same +rate, or in the same degree; yet in the long run that all undergo +modification to some extent. The extinction of old forms is the almost +inevitable consequence of the production of new forms. We can +understand why when a species has once disappeared it never reappears. +Groups of species increase in numbers slowly, and endure for unequal +periods of time; for the process of modification is necessarily slow, +and depends on many complex contingencies. The dominant species of the +larger dominant groups tend to leave many modified +descendants, and thus new sub-groups and groups are formed. As these +are formed, the species of the less vigorous groups, from their +inferiority inherited from a common progenitor, tend to become extinct +together, and to leave no modified offspring on the face of the earth. +But the utter extinction of a whole group of species may often be a +very slow process, from the survival of a few descendants, lingering in +protected and isolated situations. When a group has once wholly +disappeared, it does not reappear; for the link of generation has been +broken. + +We can understand how the spreading of the dominant forms of life, +which are those that oftenest vary, will in the long run tend to people +the world with allied, but modified, descendants; and these will +generally succeed in taking the places of those groups of species which +are their inferiors in the struggle for existence. Hence, after long +intervals of time, the productions of the world will appear to have +changed simultaneously. + +We can understand how it is that all the forms of life, ancient and +recent, make together one grand system; for all are connected by +generation. We can understand, from the continued tendency to +divergence of character, why the more ancient a form is, the more it +generally differs from those now living. Why ancient and extinct forms +often tend to fill up gaps between existing forms, sometimes blending +two groups previously classed as distinct into one; but more commonly +only bringing them a little closer together. The more ancient a form +is, the more often, apparently, it displays characters in some degree +intermediate between groups now distinct; for the more ancient a form +is, the more nearly it will be related to, and consequently resemble, +the common progenitor of groups, since become +widely divergent. Extinct forms are seldom directly intermediate +between existing forms; but are intermediate only by a long and +circuitous course through many extinct and very different forms. We can +clearly see why the organic remains of closely consecutive formations +are more closely allied to each other, than are those of remote +formations; for the forms are more closely linked together by +generation: we can clearly see why the remains of an intermediate +formation are intermediate in character. + +The inhabitants of each successive period in the world’s history have +beaten their predecessors in the race for life, and are, in so far, +higher in the scale of nature; and this may account for that vague yet +ill-defined sentiment, felt by many palæontologists, that organisation +on the whole has progressed. If it should hereafter be proved that +ancient animals resemble to a certain extent the embryos of more recent +animals of the same class, the fact will be intelligible. The +succession of the same types of structure within the same areas during +the later geological periods ceases to be mysterious, and is simply +explained by inheritance. + +If then the geological record be as imperfect as I believe it to be, +and it may at least be asserted that the record cannot be proved to be +much more perfect, the main objections to the theory of natural +selection are greatly diminished or disappear. On the other hand, all +the chief laws of palæontology plainly proclaim, as it seems to me, +that species have been produced by ordinary generation: old forms +having been supplanted by new and improved forms of life, produced by +the laws of variation still acting round us, and preserved by Natural +Selection. + + + + +CHAPTER XI. +GEOGRAPHICAL DISTRIBUTION. + + +Present distribution cannot be accounted for by differences in physical +conditions. Importance of barriers. Affinity of the productions of the +same continent. Centres of creation. Means of dispersal, by changes of +climate and of the level of the land, and by occasional means. +Dispersal during the Glacial period co-extensive with the world. + + +In considering the distribution of organic beings over the face of the +globe, the first great fact which strikes us is, that neither the +similarity nor the dissimilarity of the inhabitants of various regions +can be accounted for by their climatal and other physical conditions. +Of late, almost every author who has studied the subject has come to +this conclusion. The case of America alone would almost suffice to +prove its truth: for if we exclude the northern parts where the +circumpolar land is almost continuous, all authors agree that one of +the most fundamental divisions in geographical distribution is that +between the New and Old Worlds; yet if we travel over the vast American +continent, from the central parts of the United States to its extreme +southern point, we meet with the most diversified conditions; the most +humid districts, arid deserts, lofty mountains, grassy plains, forests, +marshes, lakes, and great rivers, under almost every temperature. There +is hardly a climate or condition in the Old World which cannot be +paralleled in the New—at least as closely as the same species generally +require; for it is a most rare case to find a group of organisms +confined to any small spot, having conditions peculiar in only a slight +degree; for instance, small areas in the Old World could be pointed out +hotter than any in the New World, yet these are not inhabited by a +peculiar fauna or flora. Notwithstanding this parallelism in the +conditions of the Old and New Worlds, how widely different are their +living productions! + +In the southern hemisphere, if we compare large tracts of land in +Australia, South Africa, and western South America, between latitudes +25° and 35°, we shall find parts extremely similar in all their +conditions, yet it would not be possible to point out three faunas and +floras more utterly dissimilar. Or again we may compare the productions +of South America south of lat. 35° with those north of 25°, which +consequently inhabit a considerably different climate, and they will be +found incomparably more closely related to each other, than they are to +the productions of Australia or Africa under nearly the same climate. +Analogous facts could be given with respect to the inhabitants of the +sea. + +A second great fact which strikes us in our general review is, that +barriers of any kind, or obstacles to free migration, are related in a +close and important manner to the differences between the productions +of various regions. We see this in the great difference of nearly all +the terrestrial productions of the New and Old Worlds, excepting in the +northern parts, where the land almost joins, and where, under a +slightly different climate, there might have been free migration for +the northern temperate forms, as there now is for the strictly arctic +productions. We see the same fact in the great difference between the +inhabitants of Australia, Africa, and South America under the same +latitude: for these countries are almost as much isolated from each +other as is possible. On each continent, also, we see the same fact; +for on the opposite sides of +lofty and continuous mountain-ranges, and of great deserts, and +sometimes even of large rivers, we find different productions; though +as mountain chains, deserts, etc., are not as impassable, or likely to +have endured so long as the oceans separating continents, the +differences are very inferior in degree to those characteristic of +distinct continents. + +Turning to the sea, we find the same law. No two marine faunas are more +distinct, with hardly a fish, shell, or crab in common, than those of +the eastern and western shores of South and Central America; yet these +great faunas are separated only by the narrow, but impassable, isthmus +of Panama. Westward of the shores of America, a wide space of open +ocean extends, with not an island as a halting-place for emigrants; +here we have a barrier of another kind, and as soon as this is passed +we meet in the eastern islands of the Pacific, with another and totally +distinct fauna. So that here three marine faunas range far northward +and southward, in parallel lines not far from each other, under +corresponding climates; but from being separated from each other by +impassable barriers, either of land or open sea, they are wholly +distinct. On the other hand, proceeding still further westward from the +eastern islands of the tropical parts of the Pacific, we encounter no +impassable barriers, and we have innumerable islands as halting-places, +until after travelling over a hemisphere we come to the shores of +Africa; and over this vast space we meet with no well-defined and +distinct marine faunas. Although hardly one shell, crab or fish is +common to the above-named three approximate faunas of Eastern and +Western America and the eastern Pacific islands, yet many fish range +from the Pacific into the Indian Ocean, and many shells are common to +the eastern islands of the Pacific +and the eastern shores of Africa, on almost exactly opposite meridians +of longitude. + +A third great fact, partly included in the foregoing statements, is the +affinity of the productions of the same continent or sea, though the +species themselves are distinct at different points and stations. It is +a law of the widest generality, and every continent offers innumerable +instances. Nevertheless the naturalist in travelling, for instance, +from north to south never fails to be struck by the manner in which +successive groups of beings, specifically distinct, yet clearly +related, replace each other. He hears from closely allied, yet distinct +kinds of birds, notes nearly similar, and sees their nests similarly +constructed, but not quite alike, with eggs coloured in nearly the same +manner. The plains near the Straits of Magellan are inhabited by one +species of Rhea (American ostrich), and northward the plains of La +Plata by another species of the same genus; and not by a true ostrich +or emeu, like those found in Africa and Australia under the same +latitude. On these same plains of La Plata, we see the agouti and +bizcacha, animals having nearly the same habits as our hares and +rabbits and belonging to the same order of Rodents, but they plainly +display an American type of structure. We ascend the lofty peaks of the +Cordillera and we find an alpine species of bizcacha; we look to the +waters, and we do not find the beaver or musk-rat, but the coypu and +capybara, rodents of the American type. Innumerable other instances +could be given. If we look to the islands off the American shore, +however much they may differ in geological structure, the inhabitants, +though they may be all peculiar species, are essentially American. We +may look back to past ages, as shown in the last chapter, and we find +American types then prevalent on +the American continent and in the American seas. We see in these facts +some deep organic bond, prevailing throughout space and time, over the +same areas of land and water, and independent of their physical +conditions. The naturalist must feel little curiosity, who is not led +to inquire what this bond is. + +This bond, on my theory, is simply inheritance, that cause which alone, +as far as we positively know, produces organisms quite like, or, as we +see in the case of varieties nearly like each other. The dissimilarity +of the inhabitants of different regions may be attributed to +modification through natural selection, and in a quite subordinate +degree to the direct influence of different physical conditions. The +degree of dissimilarity will depend on the migration of the more +dominant forms of life from one region into another having been +effected with more or less ease, at periods more or less remote;—on the +nature and number of the former immigrants;—and on their action and +reaction, in their mutual struggles for life;—the relation of organism +to organism being, as I have already often remarked, the most important +of all relations. Thus the high importance of barriers comes into play +by checking migration; as does time for the slow process of +modification through natural selection. Widely-ranging species, +abounding in individuals, which have already triumphed over many +competitors in their own widely-extended homes will have the best +chance of seizing on new places, when they spread into new countries. +In their new homes they will be exposed to new conditions, and will +frequently undergo further modification and improvement; and thus they +will become still further victorious, and will produce groups of +modified descendants. On this principle of inheritance with +modification, we can understand how it is that sections of genera, +whole genera, +and even families are confined to the same areas, as is so commonly and +notoriously the case. + +I believe, as was remarked in the last chapter, in no law of necessary +development. As the variability of each species is an independent +property, and will be taken advantage of by natural selection, only so +far as it profits the individual in its complex struggle for life, so +the degree of modification in different species will be no uniform +quantity. If, for instance, a number of species, which stand in direct +competition with each other, migrate in a body into a new and +afterwards isolated country, they will be little liable to +modification; for neither migration nor isolation in themselves can do +anything. These principles come into play only by bringing organisms +into new relations with each other, and in a lesser degree with the +surrounding physical conditions. As we have seen in the last chapter +that some forms have retained nearly the same character from an +enormously remote geological period, so certain species have migrated +over vast spaces, and have not become greatly modified. + +On these views, it is obvious, that the several species of the same +genus, though inhabiting the most distant quarters of the world, must +originally have proceeded from the same source, as they have descended +from the same progenitor. In the case of those species, which have +undergone during whole geological periods but little modification, +there is not much difficulty in believing that they may have migrated +from the same region; for during the vast geographical and climatal +changes which will have supervened since ancient times, almost any +amount of migration is possible. But in many other cases, in which we +have reason to believe that the species of a genus have been produced +within comparatively recent times, there is great difficulty on this +head. It +is also obvious that the individuals of the same species, though now +inhabiting distant and isolated regions, must have proceeded from one +spot, where their parents were first produced: for, as explained in the +last chapter, it is incredible that individuals identically the same +should ever have been produced through natural selection from parents +specifically distinct. + +We are thus brought to the question which has been largely discussed by +naturalists, namely, whether species have been created at one or more +points of the earth’s surface. Undoubtedly there are very many cases of +extreme difficulty, in understanding how the same species could +possibly have migrated from some one point to the several distant and +isolated points, where now found. Nevertheless the simplicity of the +view that each species was first produced within a single region +captivates the mind. He who rejects it, rejects the _vera causa_ of +ordinary generation with subsequent migration, and calls in the agency +of a miracle. It is universally admitted, that in most cases the area +inhabited by a species is continuous; and when a plant or animal +inhabits two points so distant from each other, or with an interval of +such a nature, that the space could not be easily passed over by +migration, the fact is given as something remarkable and exceptional. +The capacity of migrating across the sea is more distinctly limited in +terrestrial mammals, than perhaps in any other organic beings; and, +accordingly, we find no inexplicable cases of the same mammal +inhabiting distant points of the world. No geologist will feel any +difficulty in such cases as Great Britain having been formerly united +to Europe, and consequently possessing the same quadrupeds. But if the +same species can be produced at two separate points, why do we not find +a single mammal common to Europe and Australia or South America? The +conditions of life are +nearly the same, so that a multitude of European animals and plants +have become naturalised in America and Australia; and some of the +aboriginal plants are identically the same at these distant points of +the northern and southern hemispheres? The answer, as I believe, is, +that mammals have not been able to migrate, whereas some plants, from +their varied means of dispersal, have migrated across the vast and +broken interspace. The great and striking influence which barriers of +every kind have had on distribution, is intelligible only on the view +that the great majority of species have been produced on one side +alone, and have not been able to migrate to the other side. Some few +families, many sub-families, very many genera, and a still greater +number of sections of genera are confined to a single region; and it +has been observed by several naturalists, that the most natural genera, +or those genera in which the species are most closely related to each +other, are generally local, or confined to one area. What a strange +anomaly it would be, if, when coming one step lower in the series, to +the individuals of the same species, a directly opposite rule +prevailed; and species were not local, but had been produced in two or +more distinct areas! + +Hence it seems to me, as it has to many other naturalists, that the +view of each species having been produced in one area alone, and having +subsequently migrated from that area as far as its powers of migration +and subsistence under past and present conditions permitted, is the +most probable. Undoubtedly many cases occur, in which we cannot explain +how the same species could have passed from one point to the other. But +the geographical and climatal changes, which have certainly occurred +within recent geological times, must have interrupted or rendered +discontinuous the formerly continuous range of many species. So that we +are reduced to consider whether the exceptions to +continuity of range are so numerous and of so grave a nature, that we +ought to give up the belief, rendered probable by general +considerations, that each species has been produced within one area, +and has migrated thence as far as it could. It would be hopelessly +tedious to discuss all the exceptional cases of the same species, now +living at distant and separated points; nor do I for a moment pretend +that any explanation could be offered of many such cases. But after +some preliminary remarks, I will discuss a few of the most striking +classes of facts; namely, the existence of the same species on the +summits of distant mountain-ranges, and at distant points in the arctic +and antarctic regions; and secondly (in the following chapter), the +wide distribution of freshwater productions; and thirdly, the +occurrence of the same terrestrial species on islands and on the +mainland, though separated by hundreds of miles of open sea. If the +existence of the same species at distant and isolated points of the +earth’s surface, can in many instances be explained on the view of each +species having migrated from a single birthplace; then, considering our +ignorance with respect to former climatal and geographical changes and +various occasional means of transport, the belief that this has been +the universal law, seems to me incomparably the safest. + +In discussing this subject, we shall be enabled at the same time to +consider a point equally important for us, namely, whether the several +distinct species of a genus, which on my theory have all descended from +a common progenitor, can have migrated (undergoing modification during +some part of their migration) from the area inhabited by their +progenitor. If it can be shown to be almost invariably the case, that a +region, of which most of its inhabitants are closely related to, or +belong to the same genera with the species of a second region, +has probably received at some former period immigrants from this other +region, my theory will be strengthened; for we can clearly understand, +on the principle of modification, why the inhabitants of a region +should be related to those of another region, whence it has been +stocked. A volcanic island, for instance, upheaved and formed at the +distance of a few hundreds of miles from a continent, would probably +receive from it in the course of time a few colonists, and their +descendants, though modified, would still be plainly related by +inheritance to the inhabitants of the continent. Cases of this nature +are common, and are, as we shall hereafter more fully see, inexplicable +on the theory of independent creation. This view of the relation of +species in one region to those in another, does not differ much (by +substituting the word variety for species) from that lately advanced in +an ingenious paper by Mr. Wallace, in which he concludes, that “every +species has come into existence coincident both in space and time with +a pre-existing closely allied species.” And I now know from +correspondence, that this coincidence he attributes to generation with +modification. + +The previous remarks on “single and multiple centres of creation” do +not directly bear on another allied question,—namely whether all the +individuals of the same species have descended from +a single pair, or single hermaphrodite, or whether, as some authors +suppose, from many individuals simultaneously created. With those +organic beings which never intercross (if such exist), the species, on +my theory, must have descended from a succession of improved varieties, +which will never have blended with other individuals or varieties, but +will have supplanted each other; so that, at each successive stage of +modification and improvement, all the individuals of each variety will +have descended from a single parent. But in the majority of cases, +namely, with all organisms which habitually unite for each birth, or +which often intercross, I believe that during the slow process of +modification the individuals of the species will have been kept nearly +uniform by intercrossing; so that many individuals will have gone on +simultaneously changing, and the whole amount of modification will not +have been due, at each stage, to descent from a single parent. To +illustrate what I mean: our English racehorses differ slightly from the +horses of every other breed; but they do not owe their difference and +superiority to descent from any single pair, but to continued care in +selecting and training many individuals during many generations. + +Before discussing the three classes of facts, which I have selected as +presenting the greatest amount of difficulty on the theory of “single +centres of creation,” I must say a few words on the means of dispersal. + +_Means of Dispersal_.—Sir C. Lyell and other authors have ably treated +this subject. I can give here only the briefest abstract of the more +important facts. Change of climate must have had a powerful influence +on migration: a region when its climate was different may have been a +high road for migration, but now be impassable; I shall, however, +presently have to discuss this branch of the subject in some detail. +Changes of level in the land must also have been highly influential: a +narrow isthmus now separates two marine faunas; submerge it, or let it +formerly have been submerged, and the two faunas will now blend or may +formerly have blended: where the sea now extends, land may at a former +period have connected islands or possibly even continents together, and +thus have allowed terrestrial productions to pass from one to the +other. +No geologist will dispute that great mutations of level have occurred +within the period of existing organisms. Edward Forbes insisted that +all the islands in the Atlantic must recently have been connected with +Europe or Africa, and Europe likewise with America. Other authors have +thus hypothetically bridged over every ocean, and have united almost +every island to some mainland. If indeed the arguments used by Forbes +are to be trusted, it must be admitted that scarcely a single island +exists which has not recently been united to some continent. This view +cuts the Gordian knot of the dispersal of the same species to the most +distant points, and removes many a difficulty: but to the best of my +judgment we are not authorized in admitting such enormous geographical +changes within the period of existing species. It seems to me that we +have abundant evidence of great oscillations of level in our +continents; but not of such vast changes in their position and +extension, as to have united them within the recent period to each +other and to the several intervening oceanic islands. I freely admit +the former existence of many islands, now buried beneath the sea, which +may have served as halting places for plants and for many animals +during their migration. In the coral-producing oceans such sunken +islands are now marked, as I believe, by rings of coral or atolls +standing over them. Whenever it is fully admitted, as I believe it will +some day be, that each species has proceeded from a single birthplace, +and when in the course of time we know something definite about the +means of distribution, we shall be enabled to speculate with security +on the former extension of the land. But I do not believe that it will +ever be proved that within the recent period continents which are now +quite separate, have been continuously, or almost continuously, united +with each other, and with the many existing oceanic islands. Several +facts in distribution,—such as the great difference in the marine +faunas on the opposite sides of almost every continent,—the close +relation of the tertiary inhabitants of several lands and even seas to +their present inhabitants,—a certain degree of relation (as we shall +hereafter see) between the distribution of mammals and the depth of the +sea,—these and other such facts seem to me opposed to the admission of +such prodigious geographical revolutions within the recent period, as +are necessitated on the view advanced by Forbes and admitted by his +many followers. The nature and relative proportions of the inhabitants +of oceanic islands likewise seem to me opposed to the belief of their +former continuity with continents. Nor does their almost universally +volcanic composition favour the admission that they are the wrecks of +sunken continents;—if they had originally existed as mountain-ranges on +the land, some at least of the islands would have been formed, like +other mountain-summits, of granite, metamorphic schists, old +fossiliferous or other such rocks, instead of consisting of mere piles +of volcanic matter. + +I must now say a few words on what are called accidental means, but +which more properly might be called occasional means of distribution. I +shall here confine myself to plants. In botanical works, this or that +plant is stated to be ill adapted for wide dissemination; but for +transport across the sea, the greater or less facilities may be said to +be almost wholly unknown. Until I tried, with Mr. Berkeley’s aid, a few +experiments, it was not even known how far seeds could resist the +injurious action of sea-water. To my surprise I found that out of 87 +kinds, 64 germinated after an immersion of 28 days, and a few survived +an immersion of 137 days. +For convenience sake I chiefly tried small seeds, without the capsule +or fruit; and as all of these sank in a few days, they could not be +floated across wide spaces of the sea, whether or not they were injured +by the salt-water. Afterwards I tried some larger fruits, capsules, +etc., and some of these floated for a long time. It is well known what +a difference there is in the buoyancy of green and seasoned timber; and +it occurred to me that floods might wash down plants or branches, and +that these might be dried on the banks, and then by a fresh rise in the +stream be washed into the sea. Hence I was led to dry stems and +branches of 94 plants with ripe fruit, and to place them on sea water. +The majority sank quickly, but some which whilst green floated for a +very short time, when dried floated much longer; for instance, ripe +hazel-nuts sank immediately, but when dried, they floated for 90 days +and afterwards when planted they germinated; an asparagus plant with +ripe berries floated for 23 days, when dried it floated for 85 days, +and the seeds afterwards germinated: the ripe seeds of Helosciadium +sank in two days, when dried they floated for above 90 days, and +afterwards germinated. Altogether out of the 94 dried plants, 18 +floated for above 28 days, and some of the 18 floated for a very much +longer period. So that as 64/87 seeds germinated after an immersion of +28 days; and as 18/94 plants with ripe fruit (but not all the same +species as in the foregoing experiment) floated, after being dried, for +above 28 days, as far as we may infer anything from these scanty facts, +we may conclude that the seeds of 14/100 plants of any country might be +floated by sea-currents during 28 days, and would retain their power of +germination. In Johnston’s Physical Atlas, the average rate of the +several Atlantic currents is 33 miles per diem (some currents running +at the rate of 60 miles +per diem); on this average, the seeds of 14/100 plants belonging to one +country might be floated across 924 miles of sea to another country; +and when stranded, if blown to a favourable spot by an inland gale, +they would germinate. + +Subsequently to my experiments, M. Martens tried similar ones, but in a +much better manner, for he placed the seeds in a box in the actual sea, +so that they were alternately wet and exposed to the air like really +floating plants. He tried 98 seeds, mostly different from mine; but he +chose many large fruits and likewise seeds from plants which live near +the sea; and this would have favoured the average length of their +flotation and of their resistance to the injurious action of the +salt-water. On the other hand he did not previously dry the plants or +branches with the fruit; and this, as we have seen, would have caused +some of them to have floated much longer. The result was that 18/98 of +his seeds floated for 42 days, and were then capable of germination. +But I do not doubt that plants exposed to the waves would float for a +less time than those protected from violent movement as in our +experiments. Therefore it would perhaps be safer to assume that the +seeds of about 10/100 plants of a flora, after having been dried, could +be floated across a space of sea 900 miles in width, and would then +germinate. The fact of the larger fruits often floating longer than the +small, is interesting; as plants with large seeds or fruit could hardly +be transported by any other means; and Alph. de Candolle has shown that +such plants generally have restricted ranges. + +But seeds may be occasionally transported in another manner. Drift +timber is thrown up on most islands, even on those in the midst of the +widest oceans; and the natives of the coral-islands in the Pacific, +procure +stones for their tools, solely from the roots of drifted trees, these +stones being a valuable royal tax. I find on examination, that when +irregularly shaped stones are embedded in the roots of trees, small +parcels of earth are very frequently enclosed in their interstices and +behind them,—so perfectly that not a particle could be washed away in +the longest transport: out of one small portion of earth thus +_completely_ enclosed by wood in an oak about 50 years old, three +dicotyledonous plants germinated: I am certain of the accuracy of this +observation. Again, I can show that the carcasses of birds, when +floating on the sea, sometimes escape being immediately devoured; and +seeds of many kinds in the crops of floating birds long retain their +vitality: peas and vetches, for instance, are killed by even a few +days’ immersion in sea-water; but some taken out of the crop of a +pigeon, which had floated on artificial salt-water for 30 days, to my +surprise nearly all germinated. + +Living birds can hardly fail to be highly effective agents in the +transportation of seeds. I could give many facts showing how frequently +birds of many kinds are blown by gales to vast distances across the +ocean. We may I think safely assume that under such circumstances their +rate of flight would often be 35 miles an hour; and some authors have +given a far higher estimate. I have never seen an instance of +nutritious seeds passing through the intestines of a bird; but hard +seeds of fruit will pass uninjured through even the digestive organs of +a turkey. In the course of two months, I picked up in my garden 12 +kinds of seeds, out of the excrement of small birds, and these seemed +perfect, and some of them, which I tried, germinated. But the following +fact is more important: the crops of birds do not secrete gastric +juice, and do not in the +least injure, as I know by trial, the germination of seeds; now after a +bird has found and devoured a large supply of food, it is positively +asserted that all the grains do not pass into the gizzard for 12 or +even 18 hours. A bird in this interval might easily be blown to the +distance of 500 miles, and hawks are known to look out for tired birds, +and the contents of their torn crops might thus readily get scattered. +Mr. Brent informs me that a friend of his had to give up flying +carrier-pigeons from France to England, as the hawks on the English +coast destroyed so many on their arrival. Some hawks and owls bolt +their prey whole, and after an interval of from twelve to twenty hours, +disgorge pellets, which, as I know from experiments made in the +Zoological Gardens, include seeds capable of germination. Some seeds of +the oat, wheat, millet, canary, hemp, clover, and beet germinated after +having been from twelve to twenty-one hours in the stomachs of +different birds of prey; and two seeds of beet grew after having been +thus retained for two days and fourteen hours. Freshwater fish, I find, +eat seeds of many land and water plants: fish are frequently devoured +by birds, and thus the seeds might be transported from place to place. +I forced many kinds of seeds into the stomachs of dead fish, and then +gave their bodies to fishing-eagles, storks, and pelicans; these birds +after an interval of many hours, either rejected the seeds in pellets +or passed them in their excrement; and several of these seeds retained +their power of germination. Certain seeds, however, were always killed +by this process. + +Although the beaks and feet of birds are generally quite clean, I can +show that earth sometimes adheres to them: in one instance I removed +twenty-two grains of dry argillaceous earth from one foot of a +partridge, and in this earth there was a pebble quite as large as +the seed of a vetch. Thus seeds might occasionally be transported to +great distances; for many facts could be given showing that soil almost +everywhere is charged with seeds. Reflect for a moment on the millions +of quails which annually cross the Mediterranean; and can we doubt that +the earth adhering to their feet would sometimes include a few minute +seeds? But I shall presently have to recur to this subject. + +As icebergs are known to be sometimes loaded with earth and stones, and +have even carried brushwood, bones, and the nest of a land-bird, I can +hardly doubt that they must occasionally have transported seeds from +one part to another of the arctic and antarctic regions, as suggested +by Lyell; and during the Glacial period from one part of the now +temperate regions to another. In the Azores, from the large number of +the species of plants common to Europe, in comparison with the plants +of other oceanic islands nearer to the mainland, and (as remarked by +Mr. H. C. Watson) from the somewhat northern character of the flora in +comparison with the latitude, I suspected that these islands had been +partly stocked by ice-borne seeds, during the Glacial epoch. At my +request Sir C. Lyell wrote to M. Hartung to inquire whether he had +observed erratic boulders on these islands, and he answered that he had +found large fragments of granite and other rocks, which do not occur in +the archipelago. Hence we may safely infer that icebergs formerly +landed their rocky burthens on the shores of these mid-ocean islands, +and it is at least possible that they may have brought thither the +seeds of northern plants. + +Considering that the several above means of transport, and that several +other means, which without doubt remain to be discovered, have been in +action year after year, for centuries and tens of thousands of +years, it would I think be a marvellous fact if many plants had not +thus become widely transported. These means of transport are sometimes +called accidental, but this is not strictly correct: the currents of +the sea are not accidental, nor is the direction of prevalent gales of +wind. It should be observed that scarcely any means of transport would +carry seeds for very great distances; for seeds do not retain their +vitality when exposed for a great length of time to the action of +seawater; nor could they be long carried in the crops or intestines of +birds. These means, however, would suffice for occasional transport +across tracts of sea some hundred miles in breadth, or from island to +island, or from a continent to a neighbouring island, but not from one +distant continent to another. The floras of distant continents would +not by such means become mingled in any great degree; but would remain +as distinct as we now see them to be. The currents, from their course, +would never bring seeds from North America to Britain, though they +might and do bring seeds from the West Indies to our western shores, +where, if not killed by so long an immersion in salt-water, they could +not endure our climate. Almost every year, one or two land-birds are +blown across the whole Atlantic Ocean, from North America to the +western shores of Ireland and England; but seeds could be transported +by these wanderers only by one means, namely, in dirt sticking to their +feet, which is in itself a rare accident. Even in this case, how small +would the chance be of a seed falling on favourable soil, and coming to +maturity! But it would be a great error to argue that because a +well-stocked island, like Great Britain, has not, as far as is known +(and it would be very difficult to prove this), received within the +last few centuries, through occasional means +of transport, immigrants from Europe or any other continent, that a +poorly-stocked island, though standing more remote from the mainland, +would not receive colonists by similar means. I do not doubt that out +of twenty seeds or animals transported to an island, even if far less +well-stocked than Britain, scarcely more than one would be so well +fitted to its new home, as to become naturalised. But this, as it seems +to me, is no valid argument against what would be effected by +occasional means of transport, during the long lapse of geological +time, whilst an island was being upheaved and formed, and before it had +become fully stocked with inhabitants. On almost bare land, with few or +no destructive insects or birds living there, nearly every seed, which +chanced to arrive, would be sure to germinate and survive. + +_Dispersal during the Glacial period_.—The identity of many plants and +animals, on mountain-summits, separated from each other by hundreds of +miles of lowlands, where the Alpine species could not possibly exist, +is one of the most striking cases known of the same species living at +distant points, without the apparent possibility of their having +migrated from one to the other. It is indeed a remarkable fact to see +so many of the same plants living on the snowy regions of the Alps or +Pyrenees, and in the extreme northern parts of Europe; but it is far +more remarkable, that the plants on the White Mountains, in the United +States of America, are all the same with those of Labrador, and nearly +all the same, as we hear from Asa Gray, with those on the loftiest +mountains of Europe. Even as long ago as 1747, such facts led Gmelin to +conclude that the same species must have been independently created at +several distinct points; and we might have remained +in this same belief, had not Agassiz and others called vivid attention +to the Glacial period, which, as we shall immediately see, affords a +simple explanation of these facts. We have evidence of almost every +conceivable kind, organic and inorganic, that within a very recent +geological period, central Europe and North America suffered under an +Arctic climate. The ruins of a house burnt by fire do not tell their +tale more plainly, than do the mountains of Scotland and Wales, with +their scored flanks, polished surfaces, and perched boulders, of the +icy streams with which their valleys were lately filled. So greatly has +the climate of Europe changed, that in Northern Italy, gigantic +moraines, left by old glaciers, are now clothed by the vine and maize. +Throughout a large part of the United States, erratic boulders, and +rocks scored by drifted icebergs and coast-ice, plainly reveal a former +cold period. + +The former influence of the glacial climate on the distribution of the +inhabitants of Europe, as explained with remarkable clearness by Edward +Forbes, is substantially as follows. But we shall follow the changes +more readily, by supposing a new glacial period to come slowly on, and +then pass away, as formerly occurred. As the cold came on, and as each +more southern zone became fitted for arctic beings and ill-fitted for +their former more temperate inhabitants, the latter would be supplanted +and arctic productions would take their places. The inhabitants of the +more temperate regions would at the same time travel southward, unless +they were stopped by barriers, in which case they would perish. The +mountains would become covered with snow and ice, and their former +Alpine inhabitants would descend to the plains. By the time that the +cold had reached its maximum, we should have a uniform arctic fauna and +flora, covering the central parts of Europe, as far +south as the Alps and Pyrenees, and even stretching into Spain. The now +temperate regions of the United States would likewise be covered by +arctic plants and animals, and these would be nearly the same with +those of Europe; for the present circumpolar inhabitants, which we +suppose to have everywhere travelled southward, are remarkably uniform +round the world. We may suppose that the Glacial period came on a +little earlier or later in North America than in Europe, so will the +southern migration there have been a little earlier or later; but this +will make no difference in the final result. + +As the warmth returned, the arctic forms would retreat northward, +closely followed up in their retreat by the productions of the more +temperate regions. And as the snow melted from the bases of the +mountains, the arctic forms would seize on the cleared and thawed +ground, always ascending higher and higher, as the warmth increased, +whilst their brethren were pursuing their northern journey. Hence, when +the warmth had fully returned, the same arctic species, which had +lately lived in a body together on the lowlands of the Old and New +Worlds, would be left isolated on distant mountain-summits (having been +exterminated on all lesser heights) and in the arctic regions of both +hemispheres. + +Thus we can understand the identity of many plants at points so +immensely remote as on the mountains of the United States and of +Europe. We can thus also understand the fact that the Alpine plants of +each mountain-range are more especially related to the arctic forms +living due north or nearly due north of them: for the migration as the +cold came on, and the re-migration on the returning warmth, will +generally have been due south and north. The Alpine plants, for +example, of Scotland, as remarked by Mr. H. C. Watson, +and those of the Pyrenees, as remarked by Ramond, are more especially +allied to the plants of northern Scandinavia; those of the United +States to Labrador; those of the mountains of Siberia to the arctic +regions of that country. These views, grounded as they are on the +perfectly well-ascertained occurrence of a former Glacial period, seem +to me to explain in so satisfactory a manner the present distribution +of the Alpine and Arctic productions of Europe and America, that when +in other regions we find the same species on distant mountain-summits, +we may almost conclude without other evidence, that a colder climate +permitted their former migration across the low intervening tracts, +since become too warm for their existence. + +If the climate, since the Glacial period, has ever been in any degree +warmer than at present (as some geologists in the United States believe +to have been the case, chiefly from the distribution of the fossil +Gnathodon), then the arctic and temperate productions will at a very +late period have marched a little further north, and subsequently have +retreated to their present homes; but I have met with no satisfactory +evidence with respect to this intercalated slightly warmer period, +since the Glacial period. + +The arctic forms, during their long southern migration and re-migration +northward, will have been exposed to nearly the same climate, and, as +is especially to be noticed, they will have kept in a body together; +consequently their mutual relations will not have been much disturbed, +and, in accordance with the principles inculcated in this volume, they +will not have been liable to much modification. But with our Alpine +productions, left isolated from the moment of the returning warmth, +first at the bases and ultimately on the summits of the mountains, the +case will have been somewhat different; +for it is not likely that all the same arctic species will have been +left on mountain ranges distant from each other, and have survived +there ever since; they will, also, in all probability have become +mingled with ancient Alpine species, which must have existed on the +mountains before the commencement of the Glacial epoch, and which +during its coldest period will have been temporarily driven down to the +plains; they will, also, have been exposed to somewhat different +climatal influences. Their mutual relations will thus have been in some +degree disturbed; consequently they will have been liable to +modification; and this we find has been the case; for if we compare the +present Alpine plants and animals of the several great European +mountain-ranges, though very many of the species are identically the +same, some present varieties, some are ranked as doubtful forms, and +some few are distinct yet closely allied or representative species. + +In illustrating what, as I believe, actually took place during the +Glacial period, I assumed that at its commencement the arctic +productions were as uniform round the polar regions as they are at the +present day. But the foregoing remarks on distribution apply not only +to strictly arctic forms, but also to many sub-arctic and to some few +northern temperate forms, for some of these are the same on the lower +mountains and on the plains of North America and Europe; and it may be +reasonably asked how I account for the necessary degree of uniformity +of the sub-arctic and northern temperate forms round the world, at the +commencement of the Glacial period. At the present day, the sub-arctic +and northern temperate productions of the Old and New Worlds are +separated from each other by the Atlantic Ocean and by the extreme +northern part of the Pacific. During the Glacial period, when the +inhabitants +of the Old and New Worlds lived further southwards than at present, +they must have been still more completely separated by wider spaces of +ocean. I believe the above difficulty may be surmounted by looking to +still earlier changes of climate of an opposite nature. We have good +reason to believe that during the newer Pliocene period, before the +Glacial epoch, and whilst the majority of the inhabitants of the world +were specifically the same as now, the climate was warmer than at the +present day. Hence we may suppose that the organisms now living under +the climate of latitude 60°, during the Pliocene period lived further +north under the Polar Circle, in latitude 66°-67°; and that the +strictly arctic productions then lived on the broken land still nearer +to the pole. Now if we look at a globe, we shall see that under the +Polar Circle there is almost continuous land from western Europe, +through Siberia, to eastern America. And to this continuity of the +circumpolar land, and to the consequent freedom for intermigration +under a more favourable climate, I attribute the necessary amount of +uniformity in the sub-arctic and northern temperate productions of the +Old and New Worlds, at a period anterior to the Glacial epoch. + +Believing, from reasons before alluded to, that our continents have +long remained in nearly the same relative position, though subjected to +large, but partial oscillations of level, I am strongly inclined to +extend the above view, and to infer that during some earlier and still +warmer period, such as the older Pliocene period, a large number of the +same plants and animals inhabited the almost continuous circumpolar +land; and that these plants and animals, both in the Old and New +Worlds, began slowly to migrate southwards as the climate became less +warm, long before the commencement +of the Glacial period. We now see, as I believe, their descendants, +mostly in a modified condition, in the central parts of Europe and the +United States. On this view we can understand the relationship, with +very little identity, between the productions of North America and +Europe,—a relationship which is most remarkable, considering the +distance of the two areas, and their separation by the Atlantic Ocean. +We can further understand the singular fact remarked on by several +observers, that the productions of Europe and America during the later +tertiary stages were more closely related to each other than they are +at the present time; for during these warmer periods the northern parts +of the Old and New Worlds will have been almost continuously united by +land, serving as a bridge, since rendered impassable by cold, for the +inter-migration of their inhabitants. + +During the slowly decreasing warmth of the Pliocene period, as soon as +the species in common, which inhabited the New and Old Worlds, migrated +south of the Polar Circle, they must have been completely cut off from +each other. This separation, as far as the more temperate productions +are concerned, took place long ages ago. And as the plants and animals +migrated southward, they will have become mingled in the one great +region with the native American productions, and have had to compete +with them; and in the other great region, with those of the Old World. +Consequently we have here everything favourable for much +modification,—for far more modification than with the Alpine +productions, left isolated, within a much more recent period, on the +several mountain-ranges and on the arctic lands of the two Worlds. +Hence it has come, that when we compare the now living productions of +the temperate regions of the New and Old Worlds, we find very few +identical +species (though Asa Gray has lately shown that more plants are +identical than was formerly supposed), but we find in every great class +many forms, which some naturalists rank as geographical races, and +others as distinct species; and a host of closely allied or +representative forms which are ranked by all naturalists as +specifically distinct. + +As on the land, so in the waters of the sea, a slow southern migration +of a marine fauna, which during the Pliocene or even a somewhat earlier +period, was nearly uniform along the continuous shores of the Polar +Circle, will account, on the theory of modification, for many closely +allied forms now living in areas completely sundered. Thus, I think, we +can understand the presence of many existing and tertiary +representative forms on the eastern and western shores of temperate +North America; and the still more striking case of many closely allied +crustaceans (as described in Dana’s admirable work), of some fish and +other marine animals, in the Mediterranean and in the seas of +Japan,—areas now separated by a continent and by nearly a hemisphere of +equatorial ocean. + +These cases of relationship, without identity, of the inhabitants of +seas now disjoined, and likewise of the past and present inhabitants of +the temperate lands of North America and Europe, are inexplicable on +the theory of creation. We cannot say that they have been created +alike, in correspondence with the nearly similar physical conditions of +the areas; for if we compare, for instance, certain parts of South +America with the southern continents of the Old World, we see countries +closely corresponding in all their physical conditions, but with their +inhabitants utterly dissimilar. + +But we must return to our more immediate subject, the Glacial period. I +am convinced that Forbes’s view +may be largely extended. In Europe we have the plainest evidence of the +cold period, from the western shores of Britain to the Oural range, and +southward to the Pyrenees. We may infer, from the frozen mammals and +nature of the mountain vegetation, that Siberia was similarly affected. +Along the Himalaya, at points 900 miles apart, glaciers have left the +marks of their former low descent; and in Sikkim, Dr. Hooker saw maize +growing on gigantic ancient moraines. South of the equator, we have +some direct evidence of former glacial action in New Zealand; and the +same plants, found on widely separated mountains in this island, tell +the same story. If one account which has been published can be trusted, +we have direct evidence of glacial action in the south-eastern corner +of Australia. + +Looking to America; in the northern half, ice-borne fragments of rock +have been observed on the eastern side as far south as lat. 36°-37°, +and on the shores of the Pacific, where the climate is now so +different, as far south as lat. 46 deg; erratic boulders have, also, +been noticed on the Rocky Mountains. In the Cordillera of Equatorial +South America, glaciers once extended far below their present level. In +central Chile I was astonished at the structure of a vast mound of +detritus, about 800 feet in height, crossing a valley of the Andes; and +this I now feel convinced was a gigantic moraine, left far below any +existing glacier. Further south on both sides of the continent, from +lat. 41° to the southernmost extremity, we have the clearest evidence +of former glacial action, in huge boulders transported far from their +parent source. + +We do not know that the Glacial epoch was strictly simultaneous at +these several far distant points on opposite sides of the world. But we +have good evidence in almost every case, that the epoch was included +within +the latest geological period. We have, also, excellent evidence, that +it endured for an enormous time, as measured by years, at each point. +The cold may have come on, or have ceased, earlier at one point of the +globe than at another, but seeing that it endured for long at each, and +that it was contemporaneous in a geological sense, it seems to me +probable that it was, during a part at least of the period, actually +simultaneous throughout the world. Without some distinct evidence to +the contrary, we may at least admit as probable that the glacial action +was simultaneous on the eastern and western sides of North America, in +the Cordillera under the equator and under the warmer temperate zones, +and on both sides of the southern extremity of the continent. If this +be admitted, it is difficult to avoid believing that the temperature of +the whole world was at this period simultaneously cooler. But it would +suffice for my purpose, if the temperature was at the same time lower +along certain broad belts of longitude. + +On this view of the whole world, or at least of broad longitudinal +belts, having been simultaneously colder from pole to pole, much light +can be thrown on the present distribution of identical and allied +species. In America, Dr. Hooker has shown that between forty and fifty +of the flowering plants of Tierra del Fuego, forming no inconsiderable +part of its scanty flora, are common to Europe, enormously remote as +these two points are; and there are many closely allied species. On the +lofty mountains of equatorial America a host of peculiar species +belonging to European genera occur. On the highest mountains of Brazil, +some few European genera were found by Gardner, which do not exist in +the wide intervening hot countries. So on the Silla of Caraccas the +illustrious Humboldt long ago found species belonging +to genera characteristic of the Cordillera. On the mountains of +Abyssinia, several European forms and some few representatives of the +peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope +a very few European species, believed not to have been introduced by +man, and on the mountains, some few representative European forms are +found, which have not been discovered in the intertropical parts of +Africa. On the Himalaya, and on the isolated mountain-ranges of the +peninsula of India, on the heights of Ceylon, and on the volcanic cones +of Java, many plants occur, either identically the same or representing +each other, and at the same time representing plants of Europe, not +found in the intervening hot lowlands. A list of the genera collected +on the loftier peaks of Java raises a picture of a collection made on a +hill in Europe! Still more striking is the fact that southern +Australian forms are clearly represented by plants growing on the +summits of the mountains of Borneo. Some of these Australian forms, as +I hear from Dr. Hooker, extend along the heights of the peninsula of +Malacca, and are thinly scattered, on the one hand over India and on +the other as far north as Japan. + +On the southern mountains of Australia, Dr. F. Müller has discovered +several European species; other species, not introduced by man, occur +on the lowlands; and a long list can be given, as I am informed by Dr. +Hooker, of European genera, found in Australia, but not in the +intermediate torrid regions. In the admirable ‘Introduction to the +Flora of New Zealand,’ by Dr. Hooker, analogous and striking facts are +given in regard to the plants of that large island. Hence we see that +throughout the world, the plants growing on the more lofty mountains, +and on the temperate lowlands of the northern and southern hemispheres, +are sometimes +identically the same; but they are much oftener specifically distinct, +though related to each other in a most remarkable manner. + +This brief abstract applies to plants alone: some strictly analogous +facts could be given on the distribution of terrestrial animals. In +marine productions, similar cases occur; as an example, I may quote a +remark by the highest authority, Professor Dana, that “it is certainly +a wonderful fact that New Zealand should have a closer resemblance in +its crustacea to Great Britain, its antipode, than to any other part of +the world.” Sir J. Richardson, also, speaks of the reappearance on the +shores of New Zealand, Tasmania, etc., of northern forms of fish. Dr. +Hooker informs me that twenty-five species of Algæ are common to New +Zealand and to Europe, but have not been found in the intermediate +tropical seas. + +It should be observed that the northern species and forms found in the +southern parts of the southern hemisphere, and on the mountain-ranges +of the intertropical regions, are not arctic, but belong to the +northern temperate zones. As Mr. H. C. Watson has recently remarked, +“In receding from polar towards equatorial latitudes, the Alpine or +mountain floras really become less and less arctic.” Many of the forms +living on the mountains of the warmer regions of the earth and in the +southern hemisphere are of doubtful value, being ranked by some +naturalists as specifically distinct, by others as varieties; but some +are certainly identical, and many, though closely related to northern +forms, must be ranked as distinct species. + +Now let us see what light can be thrown on the foregoing facts, on the +belief, supported as it is by a large body of geological evidence, that +the whole world, or a large part of it, was during the Glacial period +simultaneously much +colder than at present. The Glacial period, as measured by years, must +have been very long; and when we remember over what vast spaces some +naturalised plants and animals have spread within a few centuries, this +period will have been ample for any amount of migration. As the cold +came slowly on, all the tropical plants and other productions will have +retreated from both sides towards the equator, followed in the rear by +the temperate productions, and these by the arctic; but with the latter +we are not now concerned. The tropical plants probably suffered much +extinction; how much no one can say; perhaps formerly the tropics +supported as many species as we see at the present day crowded together +at the Cape of Good Hope, and in parts of temperate Australia. As we +know that many tropical plants and animals can withstand a considerable +amount of cold, many might have escaped extermination during a moderate +fall of temperature, more especially by escaping into the warmest +spots. But the great fact to bear in mind is, that all tropical +productions will have suffered to a certain extent. On the other hand, +the temperate productions, after migrating nearer to the equator, +though they will have been placed under somewhat new conditions, will +have suffered less. And it is certain that many temperate plants, if +protected from the inroads of competitors, can withstand a much warmer +climate than their own. Hence, it seems to me possible, bearing in mind +that the tropical productions were in a suffering state and could not +have presented a firm front against intruders, that a certain number of +the more vigorous and dominant temperate forms might have penetrated +the native ranks and have reached or even crossed the equator. The +invasion would, of course, have been greatly favoured by high land, and +perhaps +by a dry climate; for Dr. Falconer informs me that it is the damp with +the heat of the tropics which is so destructive to perennial plants +from a temperate climate. On the other hand, the most humid and hottest +districts will have afforded an asylum to the tropical natives. The +mountain-ranges north-west of the Himalaya, and the long line of the +Cordillera, seem to have afforded two great lines of invasion: and it +is a striking fact, lately communicated to me by Dr. Hooker, that all +the flowering plants, about forty-six in number, common to Tierra del +Fuego and to Europe still exist in North America, which must have lain +on the line of march. But I do not doubt that some temperate +productions entered and crossed even the _lowlands_ of the tropics at +the period when the cold was most intense,—when arctic forms had +migrated some twenty-five degrees of latitude from their native country +and covered the land at the foot of the Pyrenees. At this period of +extreme cold, I believe that the climate under the equator at the level +of the sea was about the same with that now felt there at the height of +six or seven thousand feet. During this the coldest period, I suppose +that large spaces of the tropical lowlands were clothed with a mingled +tropical and temperate vegetation, like that now growing with strange +luxuriance at the base of the Himalaya, as graphically described by +Hooker. + +Thus, as I believe, a considerable number of plants, a few terrestrial +animals, and some marine productions, migrated during the Glacial +period from the northern and southern temperate zones into the +intertropical regions, and some even crossed the equator. As the warmth +returned, these temperate forms would naturally ascend the higher +mountains, being exterminated on the lowlands; those which had not +reached the equator, would re-migrate northward or southward towards +their former +homes; but the forms, chiefly northern, which had crossed the equator, +would travel still further from their homes into the more temperate +latitudes of the opposite hemisphere. Although we have reason to +believe from geological evidence that the whole body of arctic shells +underwent scarcely any modification during their long southern +migration and re-migration northward, the case may have been wholly +different with those intruding forms which settled themselves on the +intertropical mountains, and in the southern hemisphere. These being +surrounded by strangers will have had to compete with many new forms of +life; and it is probable that selected modifications in their +structure, habits, and constitutions will have profited them. Thus many +of these wanderers, though still plainly related by inheritance to +their brethren of the northern or southern hemispheres, now exist in +their new homes as well-marked varieties or as distinct species. + +It is a remarkable fact, strongly insisted on by Hooker in regard to +America, and by Alph. de Candolle in regard to Australia, that many +more identical plants and allied forms have apparently migrated from +the north to the south, than in a reversed direction. We see, however, +a few southern vegetable forms on the mountains of Borneo and +Abyssinia. I suspect that this preponderant migration from north to +south is due to the greater extent of land in the north, and to the +northern forms having existed in their own homes in greater numbers, +and having consequently been advanced through natural selection and +competition to a higher stage of perfection or dominating power, than +the southern forms. And thus, when they became commingled during the +Glacial period, the northern forms were enabled to beat the less +powerful southern forms. Just in the same manner as we see at the +present day, +that very many European productions cover the ground in La Plata, and +in a lesser degree in Australia, and have to a certain extent beaten +the natives; whereas extremely few southern forms have become +naturalised in any part of Europe, though hides, wool, and other +objects likely to carry seeds have been largely imported into Europe +during the last two or three centuries from La Plata, and during the +last thirty or forty years from Australia. Something of the same kind +must have occurred on the intertropical mountains: no doubt before the +Glacial period they were stocked with endemic Alpine forms; but these +have almost everywhere largely yielded to the more dominant forms, +generated in the larger areas and more efficient workshops of the +north. In many islands the native productions are nearly equalled or +even outnumbered by the naturalised; and if the natives have not been +actually exterminated, their numbers have been greatly reduced, and +this is the first stage towards extinction. A mountain is an island on +the land; and the intertropical mountains before the Glacial period +must have been completely isolated; and I believe that the productions +of these islands on the land yielded to those produced within the +larger areas of the north, just in the same way as the productions of +real islands have everywhere lately yielded to continental forms, +naturalised by man’s agency. + +I am far from supposing that all difficulties are removed on the view +here given in regard to the range and affinities of the allied species +which live in the northern and southern temperate zones and on the +mountains of the intertropical regions. Very many difficulties remain +to be solved. I do not pretend to indicate the exact lines and means of +migration, or the reason why certain species and not others have +migrated; +why certain species have been modified and have given rise to new +groups of forms, and others have remained unaltered. We cannot hope to +explain such facts, until we can say why one species and not another +becomes naturalised by man’s agency in a foreign land; why one ranges +twice or thrice as far, and is twice or thrice as common, as another +species within their own homes. + +I have said that many difficulties remain to be solved: some of the +most remarkable are stated with admirable clearness by Dr. Hooker in +his botanical works on the antarctic regions. These cannot be here +discussed. I will only say that as far as regards the occurrence of +identical species at points so enormously remote as Kerguelen Land, New +Zealand, and Fuegia, I believe that towards the close of the Glacial +period, icebergs, as suggested by Lyell, have been largely concerned in +their dispersal. But the existence of several quite distinct species, +belonging to genera exclusively confined to the south, at these and +other distant points of the southern hemisphere, is, on my theory of +descent with modification, a far more remarkable case of difficulty. +For some of these species are so distinct, that we cannot suppose that +there has been time since the commencement of the Glacial period for +their migration, and for their subsequent modification to the necessary +degree. The facts seem to me to indicate that peculiar and very +distinct species have migrated in radiating lines from some common +centre; and I am inclined to look in the southern, as in the northern +hemisphere, to a former and warmer period, before the commencement of +the Glacial period, when the antarctic lands, now covered with ice, +supported a highly peculiar and isolated flora. I suspect that before +this flora was exterminated by the Glacial epoch, a few forms were +widely dispersed to various points of the southern hemisphere by +occasional means of transport, and by the aid, as halting-places, of +existing and now sunken islands, and perhaps at the commencement of the +Glacial period, by icebergs. By these means, as I believe, the southern +shores of America, Australia, New Zealand have become slightly tinted +by the same peculiar forms of vegetable life. + +Sir C. Lyell in a striking passage has speculated, in language almost +identical with mine, on the effects of great alternations of climate on +geographical distribution. I believe that the world has recently felt +one of his great cycles of change; and that on this view, combined with +modification through natural selection, a multitude of facts in the +present distribution both of the same and of allied forms of life can +be explained. The living waters may be said to have flowed during one +short period from the north and from the south, and to have crossed at +the equator; but to have flowed with greater force from the north so as +to have freely inundated the south. As the tide leaves its drift in +horizontal lines, though rising higher on the shores where the tide +rises highest, so have the living waters left their living drift on our +mountain-summits, in a line gently rising from the arctic lowlands to a +great height under the equator. The various beings thus left stranded +may be compared with savage races of man, driven up and surviving in +the mountain-fastnesses of almost every land, which serve as a record, +full of interest to us, of the former inhabitants of the surrounding +lowlands. + + + + +CHAPTER XII. +GEOGRAPHICAL DISTRIBUTION—_continued_. + + +Distribution of fresh-water productions. On the inhabitants of oceanic +islands. Absence of Batrachians and of terrestrial Mammals. On the +relation of the inhabitants of islands to those of the nearest +mainland. On colonisation from the nearest source with subsequent +modification. Summary of the last and present chapters. + + +As lakes and river-systems are separated from each other by barriers of +land, it might have been thought that fresh-water productions would not +have ranged widely within the same country, and as the sea is +apparently a still more impassable barrier, that they never would have +extended to distant countries. But the case is exactly the reverse. Not +only have many fresh-water species, belonging to quite different +classes, an enormous range, but allied species prevail in a remarkable +manner throughout the world. I well remember, when first collecting in +the fresh waters of Brazil, feeling much surprise at the similarity of +the fresh-water insects, shells, etc., and at the dissimilarity of the +surrounding terrestrial beings, compared with those of Britain. + +But this power in fresh-water productions of ranging widely, though so +unexpected, can, I think, in most cases be explained by their having +become fitted, in a manner highly useful to them, for short and +frequent migrations from pond to pond, or from stream to stream; and +liability to wide dispersal would follow from this capacity as an +almost necessary consequence. We can here consider only a few cases. In +regard to +fish, I believe that the same species never occur in the fresh waters +of distant continents. But on the same continent the species often +range widely and almost capriciously; for two river-systems will have +some fish in common and some different. A few facts seem to favour the +possibility of their occasional transport by accidental means; like +that of the live fish not rarely dropped by whirlwinds in India, and +the vitality of their ova when removed from the water. But I am +inclined to attribute the dispersal of fresh-water fish mainly to +slight changes within the recent period in the level of the land, +having caused rivers to flow into each other. Instances, also, could be +given of this having occurred during floods, without any change of +level. We have evidence in the loess of the Rhine of considerable +changes of level in the land within a very recent geological period, +and when the surface was peopled by existing land and fresh-water +shells. The wide difference of the fish on opposite sides of continuous +mountain-ranges, which from an early period must have parted +river-systems and completely prevented their inosculation, seems to +lead to this same conclusion. With respect to allied fresh-water fish +occurring at very distant points of the world, no doubt there are many +cases which cannot at present be explained: but some fresh-water fish +belong to very ancient forms, and in such cases there will have been +ample time for great geographical changes, and consequently time and +means for much migration. In the second place, salt-water fish can with +care be slowly accustomed to live in fresh water; and, according to +Valenciennes, there is hardly a single group of fishes confined +exclusively to fresh water, so that we may imagine that a marine member +of a fresh-water group might travel far along the shores of the sea, +and subsequently +become modified and adapted to the fresh waters of a distant land. + +Some species of fresh-water shells have a very wide range, and allied +species, which, on my theory, are descended from a common parent and +must have proceeded from a single source, prevail throughout the world. +Their distribution at first perplexed me much, as their ova are not +likely to be transported by birds, and they are immediately killed by +sea water, as are the adults. I could not even understand how some +naturalised species have rapidly spread throughout the same country. +But two facts, which I have observed—and no doubt many others remain to +be observed—throw some light on this subject. When a duck suddenly +emerges from a pond covered with duck-weed, I have twice seen these +little plants adhering to its back; and it has happened to me, in +removing a little duck-weed from one aquarium to another, that I have +quite unintentionally stocked the one with fresh-water shells from the +other. But another agency is perhaps more effectual: I suspended a +duck’s feet, which might represent those of a bird sleeping in a +natural pond, in an aquarium, where many ova of fresh-water shells were +hatching; and I found that numbers of the extremely minute and just +hatched shells crawled on the feet, and clung to them so firmly that +when taken out of the water they could not be jarred off, though at a +somewhat more advanced age they would voluntarily drop off. These just +hatched molluscs, though aquatic in their nature, survived on the +duck’s feet, in damp air, from twelve to twenty hours; and in this +length of time a duck or heron might fly at least six or seven hundred +miles, and would be sure to alight on a pool or rivulet, if blown +across sea to an oceanic island or to any other distant point. Sir +Charles Lyell also +informs me that a Dyticus has been caught with an Ancylus (a +fresh-water shell like a limpet) firmly adhering to it; and a +water-beetle of the same family, a Colymbetes, once flew on board the +‘Beagle,’ when forty-five miles distant from the nearest land: how much +farther it might have flown with a favouring gale no one can tell. + +With respect to plants, it has long been known what enormous ranges +many fresh-water and even marsh-species have, both over continents and +to the most remote oceanic islands. This is strikingly shown, as +remarked by Alph. de Candolle, in large groups of terrestrial plants, +which have only a very few aquatic members; for these latter seem +immediately to acquire, as if in consequence, a very wide range. I +think favourable means of dispersal explain this fact. I have before +mentioned that earth occasionally, though rarely, adheres in some +quantity to the feet and beaks of birds. Wading birds, which frequent +the muddy edges of ponds, if suddenly flushed, would be the most likely +to have muddy feet. Birds of this order I can show are the greatest +wanderers, and are occasionally found on the most remote and barren +islands in the open ocean; they would not be likely to alight on the +surface of the sea, so that the dirt would not be washed off their +feet; when making land, they would be sure to fly to their natural +fresh-water haunts. I do not believe that botanists are aware how +charged the mud of ponds is with seeds: I have tried several little +experiments, but will here give only the most striking case: I took in +February three table-spoonfuls of mud from three different points, +beneath water, on the edge of a little pond; this mud when dry weighed +only 6 3/4 ounces; I kept it covered up in my study for six months, +pulling up and counting each plant as it grew; the plants were +of many kinds, and were altogether 537 in number; and yet the viscid +mud was all contained in a breakfast cup! Considering these facts, I +think it would be an inexplicable circumstance if water-birds did not +transport the seeds of fresh-water plants to vast distances, and if +consequently the range of these plants was not very great. The same +agency may have come into play with the eggs of some of the smaller +fresh-water animals. + +Other and unknown agencies probably have also played a part. I have +stated that fresh-water fish eat some kinds of seeds, though they +reject many other kinds after having swallowed them; even small fish +swallow seeds of moderate size, as of the yellow water-lily and +Potamogeton. Herons and other birds, century after century, have gone +on daily devouring fish; they then take flight and go to other waters, +or are blown across the sea; and we have seen that seeds retain their +power of germination, when rejected in pellets or in excrement, many +hours afterwards. When I saw the great size of the seeds of that fine +water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks +on this plant, I thought that its distribution must remain quite +inexplicable; but Audubon states that he found the seeds of the great +southern water-lily (probably, according to Dr. Hooker, the Nelumbium +luteum) in a heron’s stomach; although I do not know the fact, yet +analogy makes me believe that a heron flying to another pond and +getting a hearty meal of fish, would probably reject from its stomach a +pellet containing the seeds of the Nelumbium undigested; or the seeds +might be dropped by the bird whilst feeding its young, in the same way +as fish are known sometimes to be dropped. + +In considering these several means of distribution, +it should be remembered that when a pond or stream is first formed, for +instance, on a rising islet, it will be unoccupied; and a single seed +or egg will have a good chance of succeeding. Although there will +always be a struggle for life between the individuals of the species, +however few, already occupying any pond, yet as the number of kinds is +small, compared with those on the land, the competition will probably +be less severe between aquatic than between terrestrial species; +consequently an intruder from the waters of a foreign country, would +have a better chance of seizing on a place, than in the case of +terrestrial colonists. We should, also, remember that some, perhaps +many, fresh-water productions are low in the scale of nature, and that +we have reason to believe that such low beings change or become +modified less quickly than the high; and this will give longer time +than the average for the migration of the same aquatic species. We +should not forget the probability of many species having formerly +ranged as continuously as fresh-water productions ever can range, over +immense areas, and having subsequently become extinct in intermediate +regions. But the wide distribution of fresh-water plants and of the +lower animals, whether retaining the same identical form or in some +degree modified, I believe mainly depends on the wide dispersal of +their seeds and eggs by animals, more especially by fresh-water birds, +which have large powers of flight, and naturally travel from one to +another and often distant piece of water. Nature, like a careful +gardener, thus takes her seeds from a bed of a particular nature, and +drops them in another equally well fitted for them. + +_On the Inhabitants of Oceanic Islands_.—We now come to the last of the +three classes of facts, which I +have selected as presenting the greatest amount of difficulty, on the +view that all the individuals both of the same and of allied species +have descended from a single parent; and therefore have all proceeded +from a common birthplace, notwithstanding that in the course of time +they have come to inhabit distant points of the globe. I have already +stated that I cannot honestly admit Forbes’s view on continental +extensions, which, if legitimately followed out, would lead to the +belief that within the recent period all existing islands have been +nearly or quite joined to some continent. This view would remove many +difficulties, but it would not, I think, explain all the facts in +regard to insular productions. In the following remarks I shall not +confine myself to the mere question of dispersal; but shall consider +some other facts, which bear on the truth of the two theories of +independent creation and of descent with modification. + +The species of all kinds which inhabit oceanic islands are few in +number compared with those on equal continental areas: Alph. de +Candolle admits this for plants, and Wollaston for insects. If we look +to the large size and varied stations of New Zealand, extending over +780 miles of latitude, and compare its flowering plants, only 750 in +number, with those on an equal area at the Cape of Good Hope or in +Australia, we must, I think, admit that something quite independently +of any difference in physical conditions has caused so great a +difference in number. Even the uniform county of Cambridge has 847 +plants, and the little island of Anglesea 764, but a few ferns and a +few introduced plants are included in these numbers, and the comparison +in some other respects is not quite fair. We have evidence that the +barren island of Ascension aboriginally possessed under half-a-dozen +flowering +plants; yet many have become naturalised on it, as they have on New +Zealand and on every other oceanic island which can be named. In St. +Helena there is reason to believe that the naturalised plants and +animals have nearly or quite exterminated many native productions. He +who admits the doctrine of the creation of each separate species, will +have to admit, that a sufficient number of the best adapted plants and +animals have not been created on oceanic islands; for man has +unintentionally stocked them from various sources far more fully and +perfectly than has nature. + +Although in oceanic islands the number of kinds of inhabitants is +scanty, the proportion of endemic species (_i.e._ those found nowhere +else in the world) is often extremely large. If we compare, for +instance, the number of the endemic land-shells in Madeira, or of the +endemic birds in the Galapagos Archipelago, with the number found on +any continent, and then compare the area of the islands with that of +the continent, we shall see that this is true. This fact might have +been expected on my theory, for, as already explained, species +occasionally arriving after long intervals in a new and isolated +district, and having to compete with new associates, will be eminently +liable to modification, and will often produce groups of modified +descendants. But it by no means follows, that, because in an island +nearly all the species of one class are peculiar, those of another +class, or of another section of the same class, are peculiar; and this +difference seems to depend on the species which do not become modified +having immigrated with facility and in a body, so that their mutual +relations have not been much disturbed. Thus in the Galapagos Islands +nearly every land-bird, but only two out of the eleven marine birds, +are peculiar; and it is obvious that +marine birds could arrive at these islands more easily than land-birds. +Bermuda, on the other hand, which lies at about the same distance from +North America as the Galapagos Islands do from South America, and which +has a very peculiar soil, does not possess one endemic land bird; and +we know from Mr. J. M. Jones’s admirable account of Bermuda, that very +many North American birds, during their great annual migrations, visit +either periodically or occasionally this island. Madeira does not +possess one peculiar bird, and many European and African birds are +almost every year blown there, as I am informed by Mr. E. V. Harcourt. +So that these two islands of Bermuda and Madeira have been stocked by +birds, which for long ages have struggled together in their former +homes, and have become mutually adapted to each other; and when settled +in their new homes, each kind will have been kept by the others to +their proper places and habits, and will consequently have been little +liable to modification. Madeira, again, is inhabited by a wonderful +number of peculiar land-shells, whereas not one species of sea-shell is +confined to its shores: now, though we do not know how seashells are +dispersed, yet we can see that their eggs or larvæ, perhaps attached to +seaweed or floating timber, or to the feet of wading-birds, might be +transported far more easily than land-shells, across three or four +hundred miles of open sea. The different orders of insects in Madeira +apparently present analogous facts. + +Oceanic islands are sometimes deficient in certain classes, and their +places are apparently occupied by the other inhabitants; in the +Galapagos Islands reptiles, and in New Zealand gigantic wingless birds, +take the place of mammals. In the plants of the Galapagos Islands, Dr. +Hooker has shown that the proportional numbers of the different orders +are very different from +what they are elsewhere. Such cases are generally accounted for by the +physical conditions of the islands; but this explanation seems to me +not a little doubtful. Facility of immigration, I believe, has been at +least as important as the nature of the conditions. + +Many remarkable little facts could be given with respect to the +inhabitants of remote islands. For instance, in certain islands not +tenanted by mammals, some of the endemic plants have beautifully hooked +seeds; yet few relations are more striking than the adaptation of +hooked seeds for transportal by the wool and fur of quadrupeds. This +case presents no difficulty on my view, for a hooked seed might be +transported to an island by some other means; and the plant then +becoming slightly modified, but still retaining its hooked seeds, would +form an endemic species, having as useless an appendage as any +rudimentary organ,—for instance, as the shrivelled wings under the +soldered elytra of many insular beetles. Again, islands often possess +trees or bushes belonging to orders which elsewhere include only +herbaceous species; now trees, as Alph. de Candolle has shown, +generally have, whatever the cause may be, confined ranges. Hence trees +would be little likely to reach distant oceanic islands; and an +herbaceous plant, though it would have no chance of successfully +competing in stature with a fully developed tree, when established on +an island and having to compete with herbaceous plants alone, might +readily gain an advantage by growing taller and taller and overtopping +the other plants. If so, natural selection would often tend to add to +the stature of herbaceous plants when growing on an island, to whatever +order they belonged, and thus convert them first into bushes and +ultimately into trees. + +With respect to the absence of whole orders on +oceanic islands, Bory St. Vincent long ago remarked that Batrachians +(frogs, toads, newts) have never been found on any of the many islands +with which the great oceans are studded. I have taken pains to verify +this assertion, and I have found it strictly true. I have, however, +been assured that a frog exists on the mountains of the great island of +New Zealand; but I suspect that this exception (if the information be +correct) may be explained through glacial agency. This general absence +of frogs, toads, and newts on so many oceanic islands cannot be +accounted for by their physical conditions; indeed it seems that +islands are peculiarly well fitted for these animals; for frogs have +been introduced into Madeira, the Azores, and Mauritius, and have +multiplied so as to become a nuisance. But as these animals and their +spawn are known to be immediately killed by sea-water, on my view we +can see that there would be great difficulty in their transportal +across the sea, and therefore why they do not exist on any oceanic +island. But why, on the theory of creation, they should not have been +created there, it would be very difficult to explain. + +Mammals offer another and similar case. I have carefully searched the +oldest voyages, but have not finished my search; as yet I have not +found a single instance, free from doubt, of a terrestrial mammal +(excluding domesticated animals kept by the natives) inhabiting an +island situated above 300 miles from a continent or great continental +island; and many islands situated at a much less distance are equally +barren. The Falkland Islands, which are inhabited by a wolf-like fox, +come nearest to an exception; but this group cannot be considered as +oceanic, as it lies on a bank connected with the mainland; moreover, +icebergs formerly brought boulders to its western shores, and they may +have formerly transported foxes, as so frequently now happens in the +arctic regions. Yet it cannot be said that small islands will not +support small mammals, for they occur in many parts of the world on +very small islands, if close to a continent; and hardly an island can +be named on which our smaller quadrupeds have not become naturalised +and greatly multiplied. It cannot be said, on the ordinary view of +creation, that there has not been time for the creation of mammals; +many volcanic islands are sufficiently ancient, as shown by the +stupendous degradation which they have suffered and by their tertiary +strata: there has also been time for the production of endemic species +belonging to other classes; and on continents it is thought that +mammals appear and disappear at a quicker rate than other and lower +animals. Though terrestrial mammals do not occur on oceanic islands, +ærial mammals do occur on almost every island. New Zealand possesses +two bats found nowhere else in the world: Norfolk Island, the Viti +Archipelago, the Bonin Islands, the Caroline and Marianne +Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it +may be asked, has the supposed creative force produced bats and no +other mammals on remote islands? On my view this question can easily be +answered; for no terrestrial mammal can be transported across a wide +space of sea, but bats can fly across. Bats have been seen wandering by +day far over the Atlantic Ocean; and two North American species either +regularly or occasionally visit Bermuda, at the distance of 600 miles +from the mainland. I hear from Mr. Tomes, who has specially studied +this family, that many of the same species have enormous ranges, and +are found on continents and on far distant islands. Hence we have only +to suppose that such wandering species have been modified +through natural selection in their new homes in relation to their new +position, and we can understand the presence of endemic bats on +islands, with the absence of all terrestrial mammals. + +Besides the absence of terrestrial mammals in relation to the +remoteness of islands from continents, there is also a relation, to a +certain extent independent of distance, between the depth of the sea +separating an island from the neighbouring mainland, and the presence +in both of the same mammiferous species or of allied species in a more +or less modified condition. Mr. Windsor Earl has made some striking +observations on this head in regard to the great Malay Archipelago, +which is traversed near Celebes by a space of deep ocean; and this +space separates two widely distinct mammalian faunas. On either side +the islands are situated on moderately deep submarine banks, and they +are inhabited by closely allied or identical quadrupeds. No doubt some +few anomalies occur in this great archipelago, and there is much +difficulty in forming a judgment in some cases owing to the probable +naturalisation of certain mammals through man’s agency; but we shall +soon have much light thrown on the natural history of this archipelago +by the admirable zeal and researches of Mr. Wallace. I have not as yet +had time to follow up this subject in all other quarters of the world; +but as far as I have gone, the relation generally holds good. We see +Britain separated by a shallow channel from Europe, and the mammals are +the same on both sides; we meet with analogous facts on many islands +separated by similar channels from Australia. The West Indian Islands +stand on a deeply submerged bank, nearly 1000 fathoms in depth, and +here we find American forms, but the species and even the genera are +distinct. As the amount of modification in all cases depends to +a certain degree on the lapse of time, and as during changes of level +it is obvious that islands separated by shallow channels are more +likely to have been continuously united within a recent period to the +mainland than islands separated by deeper channels, we can understand +the frequent relation between the depth of the sea and the degree of +affinity of the mammalian inhabitants of islands with those of a +neighbouring continent,—an inexplicable relation on the view of +independent acts of creation. + +All the foregoing remarks on the inhabitants of oceanic +islands,—namely, the scarcity of kinds—the richness in endemic forms in +particular classes or sections of classes,—the absence of whole groups, +as of batrachians, and of terrestrial mammals notwithstanding the +presence of ærial bats,—the singular proportions of certain orders of +plants,—herbaceous forms having been developed into trees, etc.,—seem +to me to accord better with the view of occasional means of transport +having been largely efficient in the long course of time, than with the +view of all our oceanic islands having been formerly connected by +continuous land with the nearest continent; for on this latter view the +migration would probably have been more complete; and if modification +be admitted, all the forms of life would have been more equally +modified, in accordance with the paramount importance of the relation +of organism to organism. + +I do not deny that there are many and grave difficulties in +understanding how several of the inhabitants of the more remote +islands, whether still retaining the same specific form or modified +since their arrival, could have reached their present homes. But the +probability of many islands having existed as halting-places, of which +not a wreck now remains, must not be overlooked. +I will here give a single instance of one of the cases of difficulty. +Almost all oceanic islands, even the most isolated and smallest, are +inhabited by land-shells, generally by endemic species, but sometimes +by species found elsewhere. Dr. Aug. A. Gould has given several +interesting cases in regard to the land-shells of the islands of the +Pacific. Now it is notorious that land-shells are very easily killed by +salt; their eggs, at least such as I have tried, sink in sea-water and +are killed by it. Yet there must be, on my view, some unknown, but +highly efficient means for their transportal. Would the just-hatched +young occasionally crawl on and adhere to the feet of birds roosting on +the ground, and thus get transported? It occurred to me that +land-shells, when hybernating and having a membranous diaphragm over +the mouth of the shell, might be floated in chinks of drifted timber +across moderately wide arms of the sea. And I found that several +species did in this state withstand uninjured an immersion in sea-water +during seven days: one of these shells was the Helix pomatia, and after +it had again hybernated I put it in sea-water for twenty days, and it +perfectly recovered. As this species has a thick calcareous operculum, +I removed it, and when it had formed a new membranous one, I immersed +it for fourteen days in sea-water, and it recovered and crawled away: +but more experiments are wanted on this head. + +The most striking and important fact for us in regard to the +inhabitants of islands, is their affinity to those of the nearest +mainland, without being actually the same species. Numerous instances +could be given of this fact. I will give only one, that of the +Galapagos Archipelago, situated under the equator, between 500 and 600 +miles from the shores of South America. Here +almost every product of the land and water bears the unmistakeable +stamp of the American continent. There are twenty-six land birds, and +twenty-five of these are ranked by Mr. Gould as distinct species, +supposed to have been created here; yet the close affinity of most of +these birds to American species in every character, in their habits, +gestures, and tones of voice, was manifest. So it is with the other +animals, and with nearly all the plants, as shown by Dr. Hooker in his +admirable memoir on the Flora of this archipelago. The naturalist, +looking at the inhabitants of these volcanic islands in the Pacific, +distant several hundred miles from the continent, yet feels that he is +standing on American land. Why should this be so? why should the +species which are supposed to have been created in the Galapagos +Archipelago, and nowhere else, bear so plain a stamp of affinity to +those created in America? There is nothing in the conditions of life, +in the geological nature of the islands, in their height or climate, or +in the proportions in which the several classes are associated +together, which resembles closely the conditions of the South American +coast: in fact there is a considerable dissimilarity in all these +respects. On the other hand, there is a considerable degree of +resemblance in the volcanic nature of the soil, in climate, height, and +size of the islands, between the Galapagos and Cape de Verde +Archipelagos: but what an entire and absolute difference in their +inhabitants! The inhabitants of the Cape de Verde Islands are related +to those of Africa, like those of the Galapagos to America. I believe +this grand fact can receive no sort of explanation on the ordinary view +of independent creation; whereas on the view here maintained, it is +obvious that the Galapagos Islands would be likely to receive +colonists, whether by occasional means of transport or +by formerly continuous land, from America; and the Cape de Verde +Islands from Africa; and that such colonists would be liable to +modification;—the principle of inheritance still betraying their +original birthplace. + +Many analogous facts could be given: indeed it is an almost universal +rule that the endemic productions of islands are related to those of +the nearest continent, or of other near islands. The exceptions are +few, and most of them can be explained. Thus the plants of Kerguelen +Land, though standing nearer to Africa than to America, are related, +and that very closely, as we know from Dr. Hooker’s account, to those +of America: but on the view that this island has been mainly stocked by +seeds brought with earth and stones on icebergs, drifted by the +prevailing currents, this anomaly disappears. New Zealand in its +endemic plants is much more closely related to Australia, the nearest +mainland, than to any other region: and this is what might have been +expected; but it is also plainly related to South America, which, +although the next nearest continent, is so enormously remote, that the +fact becomes an anomaly. But this difficulty almost disappears on the +view that both New Zealand, South America, and other southern lands +were long ago partially stocked from a nearly intermediate though +distant point, namely from the antarctic islands, when they were +clothed with vegetation, before the commencement of the Glacial period. +The affinity, which, though feeble, I am assured by Dr. Hooker is real, +between the flora of the south-western corner of Australia and of the +Cape of Good Hope, is a far more remarkable case, and is at present +inexplicable: but this affinity is confined to the plants, and will, I +do not doubt, be some day explained. + +The law which causes the inhabitants of an archipelago, +though specifically distinct, to be closely allied to those of the +nearest continent, we sometimes see displayed on a small scale, yet in +a most interesting manner, within the limits of the same archipelago. +Thus the several islands of the Galapagos Archipelago are tenanted, as +I have elsewhere shown, in a quite marvellous manner, by very closely +related species; so that the inhabitants of each separate island, +though mostly distinct, are related in an incomparably closer degree to +each other than to the inhabitants of any other part of the world. And +this is just what might have been expected on my view, for the islands +are situated so near each other that they would almost certainly +receive immigrants from the same original source, or from each other. +But this dissimilarity between the endemic inhabitants of the islands +may be used as an argument against my views; for it may be asked, how +has it happened in the several islands situated within sight of each +other, having the same geological nature, the same height, climate, +etc., that many of the immigrants should have been differently +modified, though only in a small degree. This long appeared to me a +great difficulty: but it arises in chief part from the deeply-seated +error of considering the physical conditions of a country as the most +important for its inhabitants; whereas it cannot, I think, be disputed +that the nature of the other inhabitants, with which each has to +compete, is at least as important, and generally a far more important +element of success. Now if we look to those inhabitants of the +Galapagos Archipelago which are found in other parts of the world +(laying on one side for the moment the endemic species, which cannot be +here fairly included, as we are considering how they have come to be +modified since their arrival), we find a considerable amount +of difference in the several islands. This difference might indeed have +been expected on the view of the islands having been stocked by +occasional means of transport—a seed, for instance, of one plant having +been brought to one island, and that of another plant to another +island. Hence when in former times an immigrant settled on any one or +more of the islands, or when it subsequently spread from one island to +another, it would undoubtedly be exposed to different conditions of +life in the different islands, for it would have to compete with +different sets of organisms: a plant, for instance, would find the +best-fitted ground more perfectly occupied by distinct plants in one +island than in another, and it would be exposed to the attacks of +somewhat different enemies. If then it varied, natural selection would +probably favour different varieties in the different islands. Some +species, however, might spread and yet retain the same character +throughout the group, just as we see on continents some species +spreading widely and remaining the same. + +The really surprising fact in this case of the Galapagos Archipelago, +and in a lesser degree in some analogous instances, is that the new +species formed in the separate islands have not quickly spread to the +other islands. But the islands, though in sight of each other, are +separated by deep arms of the sea, in most cases wider than the British +Channel, and there is no reason to suppose that they have at any former +period been continuously united. The currents of the sea are rapid and +sweep across the archipelago, and gales of wind are extraordinarily +rare; so that the islands are far more effectually separated from each +other than they appear to be on a map. Nevertheless a good many +species, both those found in other parts of the world and those +confined to the archipelago, are common to +the several islands, and we may infer from certain facts that these +have probably spread from some one island to the others. But we often +take, I think, an erroneous view of the probability of closely allied +species invading each other’s territory, when put into free +intercommunication. Undoubtedly if one species has any advantage +whatever over another, it will in a very brief time wholly or in part +supplant it; but if both are equally well fitted for their own places +in nature, both probably will hold their own places and keep separate +for almost any length of time. Being familiar with the fact that many +species, naturalised through man’s agency, have spread with astonishing +rapidity over new countries, we are apt to infer that most species +would thus spread; but we should remember that the forms which become +naturalised in new countries are not generally closely allied to the +aboriginal inhabitants, but are very distinct species, belonging in a +large proportion of cases, as shown by Alph. de Candolle, to distinct +genera. In the Galapagos Archipelago, many even of the birds, though so +well adapted for flying from island to island, are distinct on each; +thus there are three closely-allied species of mocking-thrush, each +confined to its own island. Now let us suppose the mocking-thrush of +Chatham Island to be blown to Charles Island, which has its own +mocking-thrush: why should it succeed in establishing itself there? We +may safely infer that Charles Island is well stocked with its own +species, for annually more eggs are laid there than can possibly be +reared; and we may infer that the mocking-thrush peculiar to Charles +Island is at least as well fitted for its home as is the species +peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston have +communicated to me a remarkable fact bearing on this subject; namely, +that Madeira and the adjoining islet of +Porto Santo possess many distinct but representative land-shells, some +of which live in crevices of stone; and although large quantities of +stone are annually transported from Porto Santo to Madeira, yet this +latter island has not become colonised by the Porto Santo species: +nevertheless both islands have been colonised by some European +land-shells, which no doubt had some advantage over the indigenous +species. From these considerations I think we need not greatly marvel +at the endemic and representative species, which inhabit the several +islands of the Galapagos Archipelago, not having universally spread +from island to island. In many other instances, as in the several +districts of the same continent, pre-occupation has probably played an +important part in checking the commingling of species under the same +conditions of life. Thus, the south-east and south-west corners of +Australia have nearly the same physical conditions, and are united by +continuous land, yet they are inhabited by a vast number of distinct +mammals, birds, and plants. + +The principle which determines the general character of the fauna and +flora of oceanic islands, namely, that the inhabitants, when not +identically the same, yet are plainly related to the inhabitants of +that region whence colonists could most readily have been derived,—the +colonists having been subsequently modified and better fitted to their +new homes,—is of the widest application throughout nature. We see this +on every mountain, in every lake and marsh. For Alpine species, +excepting in so far as the same forms, chiefly of plants, have spread +widely throughout the world during the recent Glacial epoch, are +related to those of the surrounding lowlands;—thus we have in South +America, Alpine humming-birds, Alpine rodents, Alpine plants, etc., all +of strictly American forms, and it is obvious +that a mountain, as it became slowly upheaved, would naturally be +colonised from the surrounding lowlands. So it is with the inhabitants +of lakes and marshes, excepting in so far as great facility of +transport has given the same general forms to the whole world. We see +this same principle in the blind animals inhabiting the caves of +America and of Europe. Other analogous facts could be given. And it +will, I believe, be universally found to be true, that wherever in two +regions, let them be ever so distant, many closely allied or +representative species occur, there will likewise be found some +identical species, showing, in accordance with the foregoing view, that +at some former period there has been intercommunication or migration +between the two regions. And wherever many closely-allied species +occur, there will be found many forms which some naturalists rank as +distinct species, and some as varieties; these doubtful forms showing +us the steps in the process of modification. + +This relation between the power and extent of migration of a species, +either at the present time or at some former period under different +physical conditions, and the existence at remote points of the world of +other species allied to it, is shown in another and more general way. +Mr. Gould remarked to me long ago, that in those genera of birds which +range over the world, many of the species have very wide ranges. I can +hardly doubt that this rule is generally true, though it would be +difficult to prove it. Amongst mammals, we see it strikingly displayed +in Bats, and in a lesser degree in the Felidæ and Canidæ. We see it, if +we compare the distribution of butterflies and beetles. So it is with +most fresh-water productions, in which so many genera range over the +world, and many individual species have enormous ranges. It is not +meant that in world-ranging +genera all the species have a wide range, or even that they have on an +_average_ a wide range; but only that some of the species range very +widely; for the facility with which widely-ranging species vary and +give rise to new forms will largely determine their average range. For +instance, two varieties of the same species inhabit America and Europe, +and the species thus has an immense range; but, if the variation had +been a little greater, the two varieties would have been ranked as +distinct species, and the common range would have been greatly reduced. +Still less is it meant, that a species which apparently has the +capacity of crossing barriers and ranging widely, as in the case of +certain powerfully-winged birds, will necessarily range widely; for we +should never forget that to range widely implies not only the power of +crossing barriers, but the more important power of being victorious in +distant lands in the struggle for life with foreign associates. But on +the view of all the species of a genus having descended from a single +parent, though now distributed to the most remote points of the world, +we ought to find, and I believe as a general rule we do find, that some +at least of the species range very widely; for it is necessary that the +unmodified parent should range widely, undergoing modification during +its diffusion, and should place itself under diverse conditions +favourable for the conversion of its offspring, firstly into new +varieties and ultimately into new species. + +In considering the wide distribution of certain genera, we should bear +in mind that some are extremely ancient, and must have branched off +from a common parent at a remote epoch; so that in such cases there +will have been ample time for great climatal and geographical changes +and for accidents of transport; and consequently for the migration of +some of the species into all +quarters of the world, where they may have become slightly modified in +relation to their new conditions. There is, also, some reason to +believe from geological evidence that organisms low in the scale within +each great class, generally change at a slower rate than the higher +forms; and consequently the lower forms will have had a better chance +of ranging widely and of still retaining the same specific character. +This fact, together with the seeds and eggs of many low forms being +very minute and better fitted for distant transportation, probably +accounts for a law which has long been observed, and which has lately +been admirably discussed by Alph. de Candolle in regard to plants, +namely, that the lower any group of organisms is, the more widely it is +apt to range. + +The relations just discussed,—namely, low and slowly-changing organisms +ranging more widely than the high,—some of the species of +widely-ranging genera themselves ranging widely,—such facts, as alpine, +lacustrine, and marsh productions being related (with the exceptions +before specified) to those on the surrounding low lands and dry lands, +though these stations are so different—the very close relation of the +distinct species which inhabit the islets of the same archipelago,—and +especially the striking relation of the inhabitants of each whole +archipelago or island to those of the nearest mainland,—are, I think, +utterly inexplicable on the ordinary view of the independent creation +of each species, but are explicable on the view of colonisation from +the nearest and readiest source, together with the subsequent +modification and better adaptation of the colonists to their new homes. + +_Summary of last and present Chapters_.—In these chapters I have +endeavoured to show, that if we make due allowance for our ignorance of +the full effects of all +the changes of climate and of the level of the land, which have +certainly occurred within the recent period, and of other similar +changes which may have occurred within the same period; if we remember +how profoundly ignorant we are with respect to the many and curious +means of occasional transport,—a subject which has hardly ever been +properly experimentised on; if we bear in mind how often a species may +have ranged continuously over a wide area, and then have become extinct +in the intermediate tracts, I think the difficulties in believing that +all the individuals of the same species, wherever located, have +descended from the same parents, are not insuperable. And we are led to +this conclusion, which has been arrived at by many naturalists under +the designation of single centres of creation, by some general +considerations, more especially from the importance of barriers and +from the analogical distribution of sub-genera, genera, and families. + +With respect to the distinct species of the same genus, which on my +theory must have spread from one parent-source; if we make the same +allowances as before for our ignorance, and remember that some forms of +life change most slowly, enormous periods of time being thus granted +for their migration, I do not think that the difficulties are +insuperable; though they often are in this case, and in that of the +individuals of the same species, extremely grave. + +As exemplifying the effects of climatal changes on distribution, I have +attempted to show how important has been the influence of the modern +Glacial period, which I am fully convinced simultaneously affected the +whole world, or at least great meridional belts. As showing how +diversified are the means of occasional transport, I have discussed at +some little length the means of dispersal of fresh-water productions. + + +If the difficulties be not insuperable in admitting that in the long +course of time the individuals of the same species, and likewise of +allied species, have proceeded from some one source; then I think all +the grand leading facts of geographical distribution are explicable on +the theory of migration (generally of the more dominant forms of life), +together with subsequent modification and the multiplication of new +forms. We can thus understand the high importance of barriers, whether +of land or water, which separate our several zoological and botanical +provinces. We can thus understand the localisation of sub-genera, +genera, and families; and how it is that under different latitudes, for +instance in South America, the inhabitants of the plains and mountains, +of the forests, marshes, and deserts, are in so mysterious a manner +linked together by affinity, and are likewise linked to the extinct +beings which formerly inhabited the same continent. Bearing in mind +that the mutual relations of organism to organism are of the highest +importance, we can see why two areas having nearly the same physical +conditions should often be inhabited by very different forms of life; +for according to the length of time which has elapsed since new +inhabitants entered one region; according to the nature of the +communication which allowed certain forms and not others to enter, +either in greater or lesser numbers; according or not, as those which +entered happened to come in more or less direct competition with each +other and with the aborigines; and according as the immigrants were +capable of varying more or less rapidly, there would ensue in different +regions, independently of their physical conditions, infinitely +diversified conditions of life,—there would be an almost endless amount +of organic action and reaction,—and we should find, as we do find, some +groups of beings greatly, and some only slightly modified,—some +developed +in great force, some existing in scanty numbers—in the different great +geographical provinces of the world. + +On these same principles, we can understand, as I have endeavoured to +show, why oceanic islands should have few inhabitants, but of these a +great number should be endemic or peculiar; and why, in relation to the +means of migration, one group of beings, even within the same class, +should have all its species endemic, and another group should have all +its species common to other quarters of the world. We can see why whole +groups of organisms, as batrachians and terrestrial mammals, should be +absent from oceanic islands, whilst the most isolated islands possess +their own peculiar species of ærial mammals or bats. We can see why +there should be some relation between the presence of mammals, in a +more or less modified condition, and the depth of the sea between an +island and the mainland. We can clearly see why all the inhabitants of +an archipelago, though specifically distinct on the several islets, +should be closely related to each other, and likewise be related, but +less closely, to those of the nearest continent or other source whence +immigrants were probably derived. We can see why in two areas, however +distant from each other, there should be a correlation, in the presence +of identical species, of varieties, of doubtful species, and of +distinct but representative species. + +As the late Edward Forbes often insisted, there is a striking +parallelism in the laws of life throughout time and space: the laws +governing the succession of forms in past times being nearly the same +with those governing at the present time the differences in different +areas. We see this in many facts. The endurance of each species and +group of species is continuous in time; for the exceptions to the rule +are so few, that they may +fairly be attributed to our not having as yet discovered in an +intermediate deposit the forms which are therein absent, but which +occur above and below: so in space, it certainly is the general rule +that the area inhabited by a single species, or by a group of species, +is continuous; and the exceptions, which are not rare, may, as I have +attempted to show, be accounted for by migration at some former period +under different conditions or by occasional means of transport, and by +the species having become extinct in the intermediate tracts. Both in +time and space, species and groups of species have their points of +maximum development. Groups of species, belonging either to a certain +period of time, or to a certain area, are often characterised by +trifling characters in common, as of sculpture or colour. In looking to +the long succession of ages, as in now looking to distant provinces +throughout the world, we find that some organisms differ little, whilst +others belonging to a different class, or to a different order, or even +only to a different family of the same order, differ greatly. In both +time and space the lower members of each class generally change less +than the higher; but there are in both cases marked exceptions to the +rule. On my theory these several relations throughout time and space +are intelligible; for whether we look to the forms of life which have +changed during successive ages within the same quarter of the world, or +to those which have changed after having migrated into distant +quarters, in both cases the forms within each class have been connected +by the same bond of ordinary generation; and the more nearly any two +forms are related in blood, the nearer they will generally stand to +each other in time and space; in both cases the laws of variation have +been the same, and modifications have been accumulated by the same +power of natural selection. + + + + +CHAPTER XIII. +MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: +RUDIMENTARY ORGANS. + + +CLASSIFICATION, groups subordinate to groups. Natural system. Rules and +difficulties in classification, explained on the theory of descent with +modification. Classification of varieties. Descent always used in +classification. Analogical or adaptive characters. Affinities, general, +complex and radiating. Extinction separates and defines groups. +MORPHOLOGY, between members of the same class, between parts of the +same individual. EMBRYOLOGY, laws of, explained by variations not +supervening at an early age, and being inherited at a corresponding +age. RUDIMENTARY ORGANS; their origin explained. Summary. + + +From the first dawn of life, all organic beings are found to resemble +each other in descending degrees, so that they can be classed in groups +under groups. This classification is evidently not arbitrary like the +grouping of the stars in constellations. The existence of groups would +have been of simple signification, if one group had been exclusively +fitted to inhabit the land, and another the water; one to feed on +flesh, another on vegetable matter, and so on; but the case is widely +different in nature; for it is notorious how commonly members of even +the same subgroup have different habits. In our second and fourth +chapters, on Variation and on Natural Selection, I have attempted to +show that it is the widely ranging, the much diffused and common, that +is the dominant species belonging to the larger genera, which vary +most. The varieties, or incipient species, thus produced ultimately +become converted, as I believe, into new and distinct species; and +these, on the principle of inheritance, tend to produce other new and +dominant +species. Consequently the groups which are now large, and which +generally include many dominant species, tend to go on increasing +indefinitely in size. I further attempted to show that from the varying +descendants of each species trying to occupy as many and as different +places as possible in the economy of nature, there is a constant +tendency in their characters to diverge. This conclusion was supported +by looking at the great diversity of the forms of life which, in any +small area, come into the closest competition, and by looking to +certain facts in naturalisation. + +I attempted also to show that there is a constant tendency in the forms +which are increasing in number and diverging in character, to supplant +and exterminate the less divergent, the less improved, and preceding +forms. I request the reader to turn to the diagram illustrating the +action, as formerly explained, of these several principles; and he will +see that the inevitable result is that the modified descendants +proceeding from one progenitor become broken up into groups subordinate +to groups. In the diagram each letter on the uppermost line may +represent a genus including several species; and all the genera on this +line form together one class, for all have descended from one ancient +but unseen parent, and, consequently, have inherited something in +common. But the three genera on the left hand have, on this same +principle, much in common, and form a sub-family, distinct from that +including the next two genera on the right hand, which diverged from a +common parent at the fifth stage of descent. These five genera have +also much, though less, in common; and they form a family distinct from +that including the three genera still further to the right hand, which +diverged at a still earlier period. And all these genera, descended +from (A), form an order distinct from the +genera descended from (I). So that we here have many species descended +from a single progenitor grouped into genera; and the genera are +included in, or subordinate to, sub-families, families, and orders, all +united into one class. Thus, the grand fact in natural history of the +subordination of group under group, which, from its familiarity, does +not always sufficiently strike us, is in my judgment fully explained. + +Naturalists try to arrange the species, genera, and families in each +class, on what is called the Natural System. But what is meant by this +system? Some authors look at it merely as a scheme for arranging +together those living objects which are most alike, and for separating +those which are most unlike; or as an artificial means for enunciating, +as briefly as possible, general propositions,—that is, by one sentence +to give the characters common, for instance, to all mammals, by another +those common to all carnivora, by another those common to the +dog-genus, and then by adding a single sentence, a full description is +given of each kind of dog. The ingenuity and utility of this system are +indisputable. But many naturalists think that something more is meant +by the Natural System; they believe that it reveals the plan of the +Creator; but unless it be specified whether order in time or space, or +what else is meant by the plan of the Creator, it seems to me that +nothing is thus added to our knowledge. Such expressions as that famous +one of Linnæus, and which we often meet with in a more or less +concealed form, that the characters do not make the genus, but that the +genus gives the characters, seem to imply that something more is +included in our classification, than mere resemblance. I believe that +something more is included; and that propinquity of descent,—the only +known cause of the similarity of organic beings,—is the bond, hidden as +it is by various degrees of modification, +which is partially revealed to us by our classifications. + +Let us now consider the rules followed in classification, and the +difficulties which are encountered on the view that classification +either gives some unknown plan of creation, or is simply a scheme for +enunciating general propositions and of placing together the forms most +like each other. It might have been thought (and was in ancient times +thought) that those parts of the structure which determined the habits +of life, and the general place of each being in the economy of nature, +would be of very high importance in classification. Nothing can be more +false. No one regards the external similarity of a mouse to a shrew, of +a dugong to a whale, of a whale to a fish, as of any importance. These +resemblances, though so intimately connected with the whole life of the +being, are ranked as merely “adaptive or analogical characters;” but to +the consideration of these resemblances we shall have to recur. It may +even be given as a general rule, that the less any part of the +organisation is concerned with special habits, the more important it +becomes for classification. As an instance: Owen, in speaking of the +dugong, says, “The generative organs being those which are most +remotely related to the habits and food of an animal, I have always +regarded as affording very clear indications of its true affinities. We +are least likely in the modifications of these organs to mistake a +merely adaptive for an essential character.” So with plants, how +remarkable it is that the organs of vegetation, on which their whole +life depends, are of little signification, excepting in the first main +divisions; whereas the organs of reproduction, with their product the +seed, are of paramount importance! + +We must not, therefore, in classifying, trust to resemblances in parts +of the organisation, however important +they may be for the welfare of the being in relation to the outer +world. Perhaps from this cause it has partly arisen, that almost all +naturalists lay the greatest stress on resemblances in organs of high +vital or physiological importance. No doubt this view of the +classificatory importance of organs which are important is generally, +but by no means always, true. But their importance for classification, +I believe, depends on their greater constancy throughout large groups +of species; and this constancy depends on such organs having generally +been subjected to less change in the adaptation of the species to their +conditions of life. That the mere physiological importance of an organ +does not determine its classificatory value, is almost shown by the one +fact, that in allied groups, in which the same organ, as we have every +reason to suppose, has nearly the same physiological value, its +classificatory value is widely different. No naturalist can have worked +at any group without being struck with this fact; and it has been most +fully acknowledged in the writings of almost every author. It will +suffice to quote the highest authority, Robert Brown, who in speaking +of certain organs in the Proteaceæ, says their generic importance, +“like that of all their parts, not only in this but, as I apprehend, in +every natural family, is very unequal, and in some cases seems to be +entirely lost.” Again in another work he says, the genera of the +Connaraceæ “differ in having one or more ovaria, in the existence or +absence of albumen, in the imbricate or valvular æstivation. Any one of +these characters singly is frequently of more than generic importance, +though here even when all taken together they appear insufficient to +separate Cnestis from Connarus.” To give an example amongst insects, in +one great division of the Hymenoptera, the antennæ, as Westwood has +remarked, are most constant in structure; +in another division they differ much, and the differences are of quite +subordinate value in classification; yet no one probably will say that +the antennæ in these two divisions of the same order are of unequal +physiological importance. Any number of instances could be given of the +varying importance for classification of the same important organ +within the same group of beings. + +Again, no one will say that rudimentary or atrophied organs are of high +physiological or vital importance; yet, undoubtedly, organs in this +condition are often of high value in classification. No one will +dispute that the rudimentary teeth in the upper jaws of young +ruminants, and certain rudimentary bones of the leg, are highly +serviceable in exhibiting the close affinity between Ruminants and +Pachyderms. Robert Brown has strongly insisted on the fact that the +rudimentary florets are of the highest importance in the classification +of the Grasses. + +Numerous instances could be given of characters derived from parts +which must be considered of very trifling physiological importance, but +which are universally admitted as highly serviceable in the definition +of whole groups. For instance, whether or not there is an open passage +from the nostrils to the mouth, the only character, according to Owen, +which absolutely distinguishes fishes and reptiles—the inflection of +the angle of the jaws in Marsupials—the manner in which the wings of +insects are folded—mere colour in certain Algæ—mere pubescence on parts +of the flower in grasses—the nature of the dermal covering, as hair or +feathers, in the Vertebrata. If the Ornithorhynchus had been covered +with feathers instead of hair, this external and trifling character +would, I think, have been considered by naturalists as important an aid +in determining the degree of affinity of this strange creature to +birds and reptiles, as an approach in structure in any one internal and +important organ. + +The importance, for classification, of trifling characters, mainly +depends on their being correlated with several other characters of more +or less importance. The value indeed of an aggregate of characters is +very evident in natural history. Hence, as has often been remarked, a +species may depart from its allies in several characters, both of high +physiological importance and of almost universal prevalence, and yet +leave us in no doubt where it should be ranked. Hence, also, it has +been found, that a classification founded on any single character, +however important that may be, has always failed; for no part of the +organisation is universally constant. The importance of an aggregate of +characters, even when none are important, alone explains, I think, that +saying of Linnæus, that the characters do not give the genus, but the +genus gives the characters; for this saying seems founded on an +appreciation of many trifling points of resemblance, too slight to be +defined. Certain plants, belonging to the Malpighiaceæ, bear perfect +and degraded flowers; in the latter, as A. de Jussieu has remarked, +“the greater number of the characters proper to the species, to the +genus, to the family, to the class, disappear, and thus laugh at our +classification.” But when Aspicarpa produced in France, during several +years, only degraded flowers, departing so wonderfully in a number of +the most important points of structure from the proper type of the +order, yet M. Richard sagaciously saw, as Jussieu observes, that this +genus should still be retained amongst the Malpighiaceæ. This case +seems to me well to illustrate the spirit with which our +classifications are sometimes necessarily founded. + +Practically when naturalists are at work, they do +not trouble themselves about the physiological value of the characters +which they use in defining a group, or in allocating any particular +species. If they find a character nearly uniform, and common to a great +number of forms, and not common to others, they use it as one of high +value; if common to some lesser number, they use it as of subordinate +value. This principle has been broadly confessed by some naturalists to +be the true one; and by none more clearly than by that excellent +botanist, Aug. St. Hilaire. If certain characters are always found +correlated with others, though no apparent bond of connexion can be +discovered between them, especial value is set on them. As in most +groups of animals, important organs, such as those for propelling the +blood, or for ærating it, or those for propagating the race, are found +nearly uniform, they are considered as highly serviceable in +classification; but in some groups of animals all these, the most +important vital organs, are found to offer characters of quite +subordinate value. + +We can see why characters derived from the embryo should be of equal +importance with those derived from the adult, for our classifications +of course include all ages of each species. But it is by no means +obvious, on the ordinary view, why the structure of the embryo should +be more important for this purpose than that of the adult, which alone +plays its full part in the economy of nature. Yet it has been strongly +urged by those great naturalists, Milne Edwards and Agassiz, that +embryonic characters are the most important of any in the +classification of animals; and this doctrine has very generally been +admitted as true. The same fact holds good with flowering plants, of +which the two main divisions have been founded on characters derived +from the embryo,—on the number and position of the embryonic +leaves or cotyledons, and on the mode of development of the plumule and +radicle. In our discussion on embryology, we shall see why such +characters are so valuable, on the view of classification tacitly +including the idea of descent. + +Our classifications are often plainly influenced by chains of +affinities. Nothing can be easier than to define a number of characters +common to all birds; but in the case of crustaceans, such definition +has hitherto been found impossible. There are crustaceans at the +opposite ends of the series, which have hardly a character in common; +yet the species at both ends, from being plainly allied to others, and +these to others, and so onwards, can be recognised as unequivocally +belonging to this, and to no other class of the Articulata. + +Geographical distribution has often been used, though perhaps not quite +logically, in classification, more especially in very large groups of +closely allied forms. Temminck insists on the utility or even necessity +of this practice in certain groups of birds; and it has been followed +by several entomologists and botanists. + +Finally, with respect to the comparative value of the various groups of +species, such as orders, sub-orders, families, sub-families, and +genera, they seem to be, at least at present, almost arbitrary. Several +of the best botanists, such as Mr. Bentham and others, have strongly +insisted on their arbitrary value. Instances could be given amongst +plants and insects, of a group of forms, first ranked by practised +naturalists as only a genus, and then raised to the rank of a +sub-family or family; and this has been done, not because further +research has detected important structural differences, at first +overlooked, but because numerous allied species, with slightly +different grades of difference, have been subsequently discovered. + + +All the foregoing rules and aids and difficulties in classification are +explained, if I do not greatly deceive myself, on the view that the +natural system is founded on descent with modification; that the +characters which naturalists consider as showing true affinity between +any two or more species, are those which have been inherited from a +common parent, and, in so far, all true classification is genealogical; +that community of descent is the hidden bond which naturalists have +been unconsciously seeking, and not some unknown plan of creation, or +the enunciation of general propositions, and the mere putting together +and separating objects more or less alike. + +But I must explain my meaning more fully. I believe that the +_arrangement_ of the groups within each class, in due subordination and +relation to the other groups, must be strictly genealogical in order to +be natural; but that the _amount_ of difference in the several branches +or groups, though allied in the same degree in blood to their common +progenitor, may differ greatly, being due to the different degrees of +modification which they have undergone; and this is expressed by the +forms being ranked under different genera, families, sections, or +orders. The reader will best understand what is meant, if he will take +the trouble of referring to the diagram in the fourth chapter. We will +suppose the letters A to L to represent allied genera, which lived +during the Silurian epoch, and these have descended from a species +which existed at an unknown anterior period. Species of three of these +genera (A, F, and I) have transmitted modified descendants to the +present day, represented by the fifteen genera (_a_14 to _z_14) on the +uppermost horizontal line. Now all these modified descendants from a +single species, are represented as related in blood or descent to the +same +degree; they may metaphorically be called cousins to the same millionth +degree; yet they differ widely and in different degrees from each +other. The forms descended from A, now broken up into two or three +families, constitute a distinct order from those descended from I, also +broken up into two families. Nor can the existing species, descended +from A, be ranked in the same genus with the parent A; or those from I, +with the parent I. But the existing genus F14 may be supposed to have +been but slightly modified; and it will then rank with the parent-genus +F; just as some few still living organic beings belong to Silurian +genera. So that the amount or value of the differences between organic +beings all related to each other in the same degree in blood, has come +to be widely different. Nevertheless their genealogical _arrangement_ +remains strictly true, not only at the present time, but at each +successive period of descent. All the modified descendants from A will +have inherited something in common from their common parent, as will +all the descendants from I; so will it be with each subordinate branch +of descendants, at each successive period. If, however, we choose to +suppose that any of the descendants of A or of I have been so much +modified as to have more or less completely lost traces of their +parentage, in this case, their places in a natural classification will +have been more or less completely lost,—as sometimes seems to have +occurred with existing organisms. All the descendants of the genus F, +along its whole line of descent, are supposed to have been but little +modified, and they yet form a single genus. But this genus, though much +isolated, will still occupy its proper intermediate position; for F +originally was intermediate in character between A and I, and the +several genera descended from these two genera will +have inherited to a certain extent their characters. This natural +arrangement is shown, as far as is possible on paper, in the diagram, +but in much too simple a manner. If a branching diagram had not been +used, and only the names of the groups had been written in a linear +series, it would have been still less possible to have given a natural +arrangement; and it is notoriously not possible to represent in a +series, on a flat surface, the affinities which we discover in nature +amongst the beings of the same group. Thus, on the view which I hold, +the natural system is genealogical in its arrangement, like a pedigree; +but the degrees of modification which the different groups have +undergone, have to be expressed by ranking them under different +so-called genera, sub-families, families, sections, orders, and +classes. + +It may be worth while to illustrate this view of classification, by +taking the case of languages. If we possessed a perfect pedigree of +mankind, a genealogical arrangement of the races of man would afford +the best classification of the various languages now spoken throughout +the world; and if all extinct languages, and all intermediate and +slowly changing dialects, had to be included, such an arrangement +would, I think, be the only possible one. Yet it might be that some +very ancient language had altered little, and had given rise to few new +languages, whilst others (owing to the spreading and subsequent +isolation and states of civilisation of the several races, descended +from a common race) had altered much, and had given rise to many new +languages and dialects. The various degrees of difference in the +languages from the same stock, would have to be expressed by groups +subordinate to groups; but the proper or even only possible arrangement +would still be genealogical; and this would be strictly natural, as +it would connect together all languages, extinct and modern, by the +closest affinities, and would give the filiation and origin of each +tongue. + +In confirmation of this view, let us glance at the classification of +varieties, which are believed or known to have descended from one +species. These are grouped under species, with sub-varieties under +varieties; and with our domestic productions, several other grades of +difference are requisite, as we have seen with pigeons. The origin of +the existence of groups subordinate to groups, is the same with +varieties as with species, namely, closeness of descent with various +degrees of modification. Nearly the same rules are followed in +classifying varieties, as with species. Authors have insisted on the +necessity of classing varieties on a natural instead of an artificial +system; we are cautioned, for instance, not to class two varieties of +the pine-apple together, merely because their fruit, though the most +important part, happens to be nearly identical; no one puts the swedish +and common turnips together, though the esculent and thickened stems +are so similar. Whatever part is found to be most constant, is used in +classing varieties: thus the great agriculturist Marshall says the +horns are very useful for this purpose with cattle, because they are +less variable than the shape or colour of the body, etc.; whereas with +sheep the horns are much less serviceable, because less constant. In +classing varieties, I apprehend if we had a real pedigree, a +genealogical classification would be universally preferred; and it has +been attempted by some authors. For we might feel sure, whether there +had been more or less modification, the principle of inheritance would +keep the forms together which were allied in the greatest number of +points. In tumbler pigeons, though some sub-varieties differ from the +others +in the important character of having a longer beak, yet all are kept +together from having the common habit of tumbling; but the short-faced +breed has nearly or quite lost this habit; nevertheless, without any +reasoning or thinking on the subject, these tumblers are kept in the +same group, because allied in blood and alike in some other respects. +If it could be proved that the Hottentot had descended from the Negro, +I think he would be classed under the Negro group, however much he +might differ in colour and other important characters from negroes. + +With species in a state of nature, every naturalist has in fact brought +descent into his classification; for he includes in his lowest grade, +or that of a species, the two sexes; and how enormously these sometimes +differ in the most important characters, is known to every naturalist: +scarcely a single fact can be predicated in common of the males and +hermaphrodites of certain cirripedes, when adult, and yet no one dreams +of separating them. The naturalist includes as one species the several +larval stages of the same individual, however much they may differ from +each other and from the adult; as he likewise includes the so-called +alternate generations of Steenstrup, which can only in a technical +sense be considered as the same individual. He includes monsters; he +includes varieties, not solely because they closely resemble the +parent-form, but because they are descended from it. He who believes +that the cowslip is descended from the primrose, or conversely, ranks +them together as a single species, and gives a single definition. As +soon as three Orchidean forms (Monochanthus, Myanthus, and Catasetum), +which had previously been ranked as three distinct genera, were known +to be sometimes produced on the same spike, they were immediately +included as a single species. +But it may be asked, what ought we to do, if it could be proved that +one species of kangaroo had been produced, by a long course of +modification, from a bear? Ought we to rank this one species with +bears, and what should we do with the other species? The supposition is +of course preposterous; and I might answer by the _argumentum ad +hominem_, and ask what should be done if a perfect kangaroo were seen +to come out of the womb of a bear? According to all analogy, it would +be ranked with bears; but then assuredly all the other species of the +kangaroo family would have to be classed under the bear genus. The +whole case is preposterous; for where there has been close descent in +common, there will certainly be close resemblance or affinity. + +As descent has universally been used in classing together the +individuals of the same species, though the males and females and larvæ +are sometimes extremely different; and as it has been used in classing +varieties which have undergone a certain, and sometimes a considerable +amount of modification, may not this same element of descent have been +unconsciously used in grouping species under genera, and genera under +higher groups, though in these cases the modification has been greater +in degree, and has taken a longer time to complete? I believe it has +thus been unconsciously used; and only thus can I understand the +several rules and guides which have been followed by our best +systematists. We have no written pedigrees; we have to make out +community of descent by resemblances of any kind. Therefore we choose +those characters which, as far as we can judge, are the least likely to +have been modified in relation to the conditions of life to which each +species has been recently exposed. Rudimentary structures on this view +are as good as, or even sometimes better than, other parts of the +organisation. We +care not how trifling a character may be—let it be the mere inflection +of the angle of the jaw, the manner in which an insect’s wing is +folded, whether the skin be covered by hair or feathers—if it prevail +throughout many and different species, especially those having very +different habits of life, it assumes high value; for we can account for +its presence in so many forms with such different habits, only by its +inheritance from a common parent. We may err in this respect in regard +to single points of structure, but when several characters, let them be +ever so trifling, occur together throughout a large group of beings +having different habits, we may feel almost sure, on the theory of +descent, that these characters have been inherited from a common +ancestor. And we know that such correlated or aggregated characters +have especial value in classification. + +We can understand why a species or a group of species may depart, in +several of its most important characteristics, from its allies, and yet +be safely classed with them. This may be safely done, and is often +done, as long as a sufficient number of characters, let them be ever so +unimportant, betrays the hidden bond of community of descent. Let two +forms have not a single character in common, yet if these extreme forms +are connected together by a chain of intermediate groups, we may at +once infer their community of descent, and we put them all into the +same class. As we find organs of high physiological importance—those +which serve to preserve life under the most diverse conditions of +existence—are generally the most constant, we attach especial value to +them; but if these same organs, in another group or section of a group, +are found to differ much, we at once value them less in our +classification. We shall hereafter, I think, clearly see why +embryological characters are of such high classificatory importance. +Geographical distribution may sometimes be brought usefully into play +in classing large and widely-distributed genera, because all the +species of the same genus, inhabiting any distinct and isolated region, +have in all probability descended from the same parents. + +We can understand, on these views, the very important distinction +between real affinities and analogical or adaptive resemblances. +Lamarck first called attention to this distinction, and he has been +ably followed by Macleay and others. The resemblance, in the shape of +the body and in the fin-like anterior limbs, between the dugong, which +is a pachydermatous animal, and the whale, and between both these +mammals and fishes, is analogical. Amongst insects there are +innumerable instances: thus Linnæus, misled by external appearances, +actually classed an homopterous insect as a moth. We see something of +the same kind even in our domestic varieties, as in the thickened stems +of the common and swedish turnip. The resemblance of the greyhound and +racehorse is hardly more fanciful than the analogies which have been +drawn by some authors between very distinct animals. On my view of +characters being of real importance for classification, only in so far +as they reveal descent, we can clearly understand why analogical or +adaptive character, although of the utmost importance to the welfare of +the being, are almost valueless to the systematist. For animals, +belonging to two most distinct lines of descent, may readily become +adapted to similar conditions, and thus assume a close external +resemblance; but such resemblances will not reveal—will rather tend to +conceal their blood-relationship to their proper lines of descent. We +can also understand the apparent paradox, that the very same characters +are analogical when one class or order is compared with another, but +give true affinities when the members of +the same class or order are compared one with another: thus the shape +of the body and fin-like limbs are only analogical when whales are +compared with fishes, being adaptations in both classes for swimming +through the water; but the shape of the body and fin-like limbs serve +as characters exhibiting true affinity between the several members of +the whale family; for these cetaceans agree in so many characters, +great and small, that we cannot doubt that they have inherited their +general shape of body and structure of limbs from a common ancestor. So +it is with fishes. + +As members of distinct classes have often been adapted by successive +slight modifications to live under nearly similar circumstances,—to +inhabit for instance the three elements of land, air, and water,—we can +perhaps understand how it is that a numerical parallelism has sometimes +been observed between the sub-groups in distinct classes. A naturalist, +struck by a parallelism of this nature in any one class, by arbitrarily +raising or sinking the value of the groups in other classes (and all +our experience shows that this valuation has hitherto been arbitrary), +could easily extend the parallelism over a wide range; and thus the +septenary, quinary, quaternary, and ternary classifications have +probably arisen. + +As the modified descendants of dominant species, belonging to the +larger genera, tend to inherit the advantages, which made the groups to +which they belong large and their parents dominant, they are almost +sure to spread widely, and to seize on more and more places in the +economy of nature. The larger and more dominant groups thus tend to go +on increasing in size; and they consequently supplant many smaller and +feebler groups. Thus we can account for the fact that all organisms, +recent and extinct, are included under a few great +orders, under still fewer classes, and all in one great natural system. +As showing how few the higher groups are in number, and how widely +spread they are throughout the world, the fact is striking, that the +discovery of Australia has not added a single insect belonging to a new +order; and that in the vegetable kingdom, as I learn from Dr. Hooker, +it has added only two or three orders of small size. + +In the chapter on geological succession I attempted to show, on the +principle of each group having generally diverged much in character +during the long-continued process of modification, how it is that the +more ancient forms of life often present characters in some slight +degree intermediate between existing groups. A few old and intermediate +parent-forms having occasionally transmitted to the present day +descendants but little modified, will give to us our so-called osculant +or aberrant groups. The more aberrant any form is, the greater must be +the number of connecting forms which on my theory have been +exterminated and utterly lost. And we have some evidence of aberrant +forms having suffered severely from extinction, for they are generally +represented by extremely few species; and such species as do occur are +generally very distinct from each other, which again implies +extinction. The genera Ornithorhynchus and Lepidosiren, for example, +would not have been less aberrant had each been represented by a dozen +species instead of by a single one; but such richness in species, as I +find after some investigation, does not commonly fall to the lot of +aberrant genera. We can, I think, account for this fact only by looking +at aberrant forms as failing groups conquered by more successful +competitors, with a few members preserved by some unusual coincidence +of favourable circumstances. + +Mr. Waterhouse has remarked that, when a member +belonging to one group of animals exhibits an affinity to a quite +distinct group, this affinity in most cases is general and not special: +thus, according to Mr. Waterhouse, of all Rodents, the bizcacha is most +nearly related to Marsupials; but in the points in which it approaches +this order, its relations are general, and not to any one marsupial +species more than to another. As the points of affinity of the bizcacha +to Marsupials are believed to be real and not merely adaptive, they are +due on my theory to inheritance in common. Therefore we must suppose +either that all Rodents, including the bizcacha, branched off from some +very ancient Marsupial, which will have had a character in some degree +intermediate with respect to all existing Marsupials; or that both +Rodents and Marsupials branched off from a common progenitor, and that +both groups have since undergone much modification in divergent +directions. On either view we may suppose that the bizcacha has +retained, by inheritance, more of the character of its ancient +progenitor than have other Rodents; and therefore it will not be +specially related to any one existing Marsupial, but indirectly to all +or nearly all Marsupials, from having partially retained the character +of their common progenitor, or of an early member of the group. On the +other hand, of all Marsupials, as Mr. Waterhouse has remarked, the +phascolomys resembles most nearly, not any one species, but the general +order of Rodents. In this case, however, it may be strongly suspected +that the resemblance is only analogical, owing to the phascolomys +having become adapted to habits like those of a Rodent. The elder De +Candolle has made nearly similar observations on the general nature of +the affinities of distinct orders of plants. + +On the principle of the multiplication and gradual divergence in +character of the species descended from +a common parent, together with their retention by inheritance of some +characters in common, we can understand the excessively complex and +radiating affinities by which all the members of the same family or +higher group are connected together. For the common parent of a whole +family of species, now broken up by extinction into distinct groups and +sub-groups, will have transmitted some of its characters, modified in +various ways and degrees, to all; and the several species will +consequently be related to each other by circuitous lines of affinity +of various lengths (as may be seen in the diagram so often referred +to), mounting up through many predecessors. As it is difficult to show +the blood-relationship between the numerous kindred of any ancient and +noble family, even by the aid of a genealogical tree, and almost +impossible to do this without this aid, we can understand the +extraordinary difficulty which naturalists have experienced in +describing, without the aid of a diagram, the various affinities which +they perceive between the many living and extinct members of the same +great natural class. + +Extinction, as we have seen in the fourth chapter, has played an +important part in defining and widening the intervals between the +several groups in each class. We may thus account even for the +distinctness of whole classes from each other—for instance, of birds +from all other vertebrate animals—by the belief that many ancient forms +of life have been utterly lost, through which the early progenitors of +birds were formerly connected with the early progenitors of the other +vertebrate classes. There has been less entire extinction of the forms +of life which once connected fishes with batrachians. There has been +still less in some other classes, as in that of the Crustacea, for here +the most wonderfully diverse forms are still tied +together by a long, but broken, chain of affinities. Extinction has +only separated groups: it has by no means made them; for if every form +which has ever lived on this earth were suddenly to reappear, though it +would be quite impossible to give definitions by which each group could +be distinguished from other groups, as all would blend together by +steps as fine as those between the finest existing varieties, +nevertheless a natural classification, or at least a natural +arrangement, would be possible. We shall see this by turning to the +diagram: the letters, A to L, may represent eleven Silurian genera, +some of which have produced large groups of modified descendants. Every +intermediate link between these eleven genera and their primordial +parent, and every intermediate link in each branch and sub-branch of +their descendants, may be supposed to be still alive; and the links to +be as fine as those between the finest varieties. In this case it would +be quite impossible to give any definition by which the several members +of the several groups could be distinguished from their more immediate +parents; or these parents from their ancient and unknown progenitor. +Yet the natural arrangement in the diagram would still hold good; and, +on the principle of inheritance, all the forms descended from A, or +from I, would have something in common. In a tree we can specify this +or that branch, though at the actual fork the two unite and blend +together. We could not, as I have said, define the several groups; but +we could pick out types, or forms, representing most of the characters +of each group, whether large or small, and thus give a general idea of +the value of the differences between them. This is what we should be +driven to, if we were ever to succeed in collecting all the forms in +any class which have lived throughout all time and space. We shall +certainly never succeed in making +so perfect a collection: nevertheless, in certain classes, we are +tending in this direction; and Milne Edwards has lately insisted, in an +able paper, on the high importance of looking to types, whether or not +we can separate and define the groups to which such types belong. + +Finally, we have seen that natural selection, which results from the +struggle for existence, and which almost inevitably induces extinction +and divergence of character in the many descendants from one dominant +parent-species, explains that great and universal feature in the +affinities of all organic beings, namely, their subordination in group +under group. We use the element of descent in classing the individuals +of both sexes and of all ages, although having few characters in +common, under one species; we use descent in classing acknowledged +varieties, however different they may be from their parent; and I +believe this element of descent is the hidden bond of connexion which +naturalists have sought under the term of the Natural System. On this +idea of the natural system being, in so far as it has been perfected, +genealogical in its arrangement, with the grades of difference between +the descendants from a common parent, expressed by the terms genera, +families, orders, etc., we can understand the rules which we are +compelled to follow in our classification. We can understand why we +value certain resemblances far more than others; why we are permitted +to use rudimentary and useless organs, or others of trifling +physiological importance; why, in comparing one group with a distinct +group, we summarily reject analogical or adaptive characters, and yet +use these same characters within the limits of the same group. We can +clearly see how it is that all living and extinct forms can be grouped +together in one great system; and how the several members of each class +are connected together by the most complex and radiating +lines of affinities. We shall never, probably, disentangle the +inextricable web of affinities between the members of any one class; +but when we have a distinct object in view, and do not look to some +unknown plan of creation, we may hope to make sure but slow progress. + +_Morphology_.—We have seen that the members of the same class, +independently of their habits of life, resemble each other in the +general plan of their organisation. This resemblance is often expressed +by the term “unity of type;” or by saying that the several parts and +organs in the different species of the class are homologous. The whole +subject is included under the general name of Morphology. This is the +most interesting department of natural history, and may be said to be +its very soul. What can be more curious than that the hand of a man, +formed for grasping, that of a mole for digging, the leg of the horse, +the paddle of the porpoise, and the wing of the bat, should all be +constructed on the same pattern, and should include the same bones, in +the same relative positions? Geoffroy St. Hilaire has insisted strongly +on the high importance of relative connexion in homologous organs: the +parts may change to almost any extent in form and size, and yet they +always remain connected together in the same order. We never find, for +instance, the bones of the arm and forearm, or of the thigh and leg, +transposed. Hence the same names can be given to the homologous bones +in widely different animals. We see the same great law in the +construction of the mouths of insects: what can be more different than +the immensely long spiral proboscis of a sphinx-moth, the curious +folded one of a bee or bug, and the great jaws of a beetle?—yet all +these organs, serving for such different +purposes, are formed by infinitely numerous modifications of an upper +lip, mandibles, and two pairs of maxillæ. Analogous laws govern the +construction of the mouths and limbs of crustaceans. So it is with the +flowers of plants. + +Nothing can be more hopeless than to attempt to explain this similarity +of pattern in members of the same class, by utility or by the doctrine +of final causes. The hopelessness of the attempt has been expressly +admitted by Owen in his most interesting work on the ‘Nature of Limbs.’ +On the ordinary view of the independent creation of each being, we can +only say that so it is;—that it has so pleased the Creator to construct +each animal and plant. + +The explanation is manifest on the theory of the natural selection of +successive slight modifications,—each modification being profitable in +some way to the modified form, but often affecting by correlation of +growth other parts of the organisation. In changes of this nature, +there will be little or no tendency to modify the original pattern, or +to transpose parts. The bones of a limb might be shortened and widened +to any extent, and become gradually enveloped in thick membrane, so as +to serve as a fin; or a webbed foot might have all its bones, or +certain bones, lengthened to any extent, and the membrane connecting +them increased to any extent, so as to serve as a wing: yet in all this +great amount of modification there will be no tendency to alter the +framework of bones or the relative connexion of the several parts. If +we suppose that the ancient progenitor, the archetype as it may be +called, of all mammals, had its limbs constructed on the existing +general pattern, for whatever purpose they served, we can at once +perceive the plain signification of the homologous construction of the +limbs throughout the whole class. So with the mouths of insects, we +have only to +suppose that their common progenitor had an upper lip, mandibles, and +two pair of maxillæ, these parts being perhaps very simple in form; and +then natural selection will account for the infinite diversity in +structure and function of the mouths of insects. Nevertheless, it is +conceivable that the general pattern of an organ might become so much +obscured as to be finally lost, by the atrophy and ultimately by the +complete abortion of certain parts, by the soldering together of other +parts, and by the doubling or multiplication of others,—variations +which we know to be within the limits of possibility. In the paddles of +the extinct gigantic sea-lizards, and in the mouths of certain +suctorial crustaceans, the general pattern seems to have been thus to a +certain extent obscured. + +There is another and equally curious branch of the present subject; +namely, the comparison not of the same part in different members of a +class, but of the different parts or organs in the same individual. +Most physiologists believe that the bones of the skull are homologous +with—that is correspond in number and in relative connexion with—the +elemental parts of a certain number of vertebræ. The anterior and +posterior limbs in each member of the vertebrate and articulate classes +are plainly homologous. We see the same law in comparing the +wonderfully complex jaws and legs in crustaceans. It is familiar to +almost every one, that in a flower the relative position of the sepals, +petals, stamens, and pistils, as well as their intimate structure, are +intelligible on the view that they consist of metamorphosed leaves, +arranged in a spire. In monstrous plants, we often get direct evidence +of the possibility of one organ being transformed into another; and we +can actually see in embryonic crustaceans and in many other animals, +and in flowers, that organs, which when mature +become extremely different, are at an early stage of growth exactly +alike. + +How inexplicable are these facts on the ordinary view of creation! Why +should the brain be enclosed in a box composed of such numerous and +such extraordinarily shaped pieces of bone? As Owen has remarked, the +benefit derived from the yielding of the separate pieces in the act of +parturition of mammals, will by no means explain the same construction +in the skulls of birds. Why should similar bones have been created in +the formation of the wing and leg of a bat, used as they are for such +totally different purposes? Why should one crustacean, which has an +extremely complex mouth formed of many parts, consequently always have +fewer legs; or conversely, those with many legs have simpler mouths? +Why should the sepals, petals, stamens, and pistils in any individual +flower, though fitted for such widely different purposes, be all +constructed on the same pattern? + +On the theory of natural selection, we can satisfactorily answer these +questions. In the vertebrata, we see a series of internal vertebræ +bearing certain processes and appendages; in the articulata, we see the +body divided into a series of segments, bearing external appendages; +and in flowering plants, we see a series of successive spiral whorls of +leaves. An indefinite repetition of the same part or organ is the +common characteristic (as Owen has observed) of all low or +little-modified forms; therefore we may readily believe that the +unknown progenitor of the vertebrata possessed many vertebræ; the +unknown progenitor of the articulata, many segments; and the unknown +progenitor of flowering plants, many spiral whorls of leaves. We have +formerly seen that parts many times repeated are eminently liable to +vary in number and structure; consequently it is quite probable that +natural selection, during a long-continued course of modification, +should have seized on a certain number of the primordially similar +elements, many times repeated, and have adapted them to the most +diverse purposes. And as the whole amount of modification will have +been effected by slight successive steps, we need not wonder at +discovering in such parts or organs, a certain degree of fundamental +resemblance, retained by the strong principle of inheritance. + +In the great class of molluscs, though we can homologise the parts of +one species with those of another and distinct species, we can indicate +but few serial homologies; that is, we are seldom enabled to say that +one part or organ is homologous with another in the same individual. +And we can understand this fact; for in molluscs, even in the lowest +members of the class, we do not find nearly so much indefinite +repetition of any one part, as we find in the other great classes of +the animal and vegetable kingdoms. + +Naturalists frequently speak of the skull as formed of metamorphosed +vertebræ: the jaws of crabs as metamorphosed legs; the stamens and +pistils of flowers as metamorphosed leaves; but it would in these cases +probably be more correct, as Professor Huxley has remarked, to speak of +both skull and vertebræ, both jaws and legs, etc.,—as having been +metamorphosed, not one from the other, but from some common element. +Naturalists, however, use such language only in a metaphorical sense: +they are far from meaning that during a long course of descent, +primordial organs of any kind—vertebræ in the one case and legs in the +other—have actually been modified into skulls or jaws. Yet so strong is +the appearance of a modification of this nature having occurred, that +naturalists can hardly avoid employing language having this plain +signification. On my view +these terms may be used literally; and the wonderful fact of the jaws, +for instance, of a crab retaining numerous characters, which they would +probably have retained through inheritance, if they had really been +metamorphosed during a long course of descent from true legs, or from +some simple appendage, is explained. + +_Embryology_.—It has already been casually remarked that certain organs +in the individual, which when mature become widely different and serve +for different purposes, are in the embryo exactly alike. The embryos, +also, of distinct animals within the same class are often strikingly +similar: a better proof of this cannot be given, than a circumstance +mentioned by Agassiz, namely, that having forgotten to ticket the +embryo of some vertebrate animal, he cannot now tell whether it be that +of a mammal, bird, or reptile. The vermiform larvæ of moths, flies, +beetles, etc., resemble each other much more closely than do the mature +insects; but in the case of larvæ, the embryos are active, and have +been adapted for special lines of life. A trace of the law of embryonic +resemblance, sometimes lasts till a rather late age: thus birds of the +same genus, and of closely allied genera, often resemble each other in +their first and second plumage; as we see in the spotted feathers in +the thrush group. In the cat tribe, most of the species are striped or +spotted in lines; and stripes can be plainly distinguished in the whelp +of the lion. We occasionally though rarely see something of this kind +in plants: thus the embryonic leaves of the ulex or furze, and the +first leaves of the phyllodineous acaceas, are pinnate or divided like +the ordinary leaves of the leguminosæ. + +The points of structure, in which the embryos of widely different +animals of the same class resemble each other, often have no direct +relation to their conditions +of existence. We cannot, for instance, suppose that in the embryos of +the vertebrata the peculiar loop-like course of the arteries near the +branchial slits are related to similar conditions,—in the young mammal +which is nourished in the womb of its mother, in the egg of the bird +which is hatched in a nest, and in the spawn of a frog under water. We +have no more reason to believe in such a relation, than we have to +believe that the same bones in the hand of a man, wing of a bat, and +fin of a porpoise, are related to similar conditions of life. No one +will suppose that the stripes on the whelp of a lion, or the spots on +the young blackbird, are of any use to these animals, or are related to +the conditions to which they are exposed. + +The case, however, is different when an animal during any part of its +embryonic career is active, and has to provide for itself. The period +of activity may come on earlier or later in life; but whenever it comes +on, the adaptation of the larva to its conditions of life is just as +perfect and as beautiful as in the adult animal. From such special +adaptations, the similarity of the larvæ or active embryos of allied +animals is sometimes much obscured; and cases could be given of the +larvæ of two species, or of two groups of species, differing quite as +much, or even more, from each other than do their adult parents. In +most cases, however, the larvæ, though active, still obey more or less +closely the law of common embryonic resemblance. Cirripedes afford a +good instance of this: even the illustrious Cuvier did not perceive +that a barnacle was, as it certainly is, a crustacean; but a glance at +the larva shows this to be the case in an unmistakeable manner. So +again the two main divisions of cirripedes, the pedunculated and +sessile, which differ widely in external appearance, have larvæ in all +their several stages barely distinguishable. + + +The embryo in the course of development generally rises in +organisation: I use this expression, though I am aware that it is +hardly possible to define clearly what is meant by the organisation +being higher or lower. But no one probably will dispute that the +butterfly is higher than the caterpillar. In some cases, however, the +mature animal is generally considered as lower in the scale than the +larva, as with certain parasitic crustaceans. To refer once again to +cirripedes: the larvæ in the first stage have three pairs of legs, a +very simple single eye, and a probosciformed mouth, with which they +feed largely, for they increase much in size. In the second stage, +answering to the chrysalis stage of butterflies, they have six pairs of +beautifully constructed natatory legs, a pair of magnificent compound +eyes, and extremely complex antennæ; but they have a closed and +imperfect mouth, and cannot feed: their function at this stage is, to +search by their well-developed organs of sense, and to reach by their +active powers of swimming, a proper place on which to become attached +and to undergo their final metamorphosis. When this is completed they +are fixed for life: their legs are now converted into prehensile +organs; they again obtain a well-constructed mouth; but they have no +antennæ, and their two eyes are now reconverted into a minute, single, +and very simple eye-spot. In this last and complete state, cirripedes +may be considered as either more highly or more lowly organised than +they were in the larval condition. But in some genera the larvæ become +developed either into hermaphrodites having the ordinary structure, or +into what I have called complemental males: and in the latter, the +development has assuredly been retrograde; for the male is a mere sack, +which lives for a short time, and is destitute of mouth, stomach, or +other organ of importance, excepting for reproduction. + + +We are so much accustomed to see differences in structure between the +embryo and the adult, and likewise a close similarity in the embryos of +widely different animals within the same class, that we might be led to +look at these facts as necessarily contingent in some manner on growth. +But there is no obvious reason why, for instance, the wing of a bat, or +the fin of a porpoise, should not have been sketched out with all the +parts in proper proportion, as soon as any structure became visible in +the embryo. And in some whole groups of animals and in certain members +of other groups, the embryo does not at any period differ widely from +the adult: thus Owen has remarked in regard to cuttle-fish, “there is +no metamorphosis; the cephalopodic character is manifested long before +the parts of the embryo are completed;” and again in spiders, “there is +nothing worthy to be called a metamorphosis.” The larvæ of insects, +whether adapted to the most diverse and active habits, or quite +inactive, being fed by their parents or placed in the midst of proper +nutriment, yet nearly all pass through a similar worm-like stage of +development; but in some few cases, as in that of Aphis, if we look to +the admirable drawings by Professor Huxley of the development of this +insect, we see no trace of the vermiform stage. + +How, then, can we explain these several facts in embryology,—namely the +very general, but not universal difference in structure between the +embryo and the adult;—of parts in the same individual embryo, which +ultimately become very unlike and serve for diverse purposes, being at +this early period of growth alike;—of embryos of different species +within the same class, generally, but not universally, resembling each +other;—of the structure of the embryo not being closely related to its +conditions of existence, except when the +embryo becomes at any period of life active and has to provide for +itself;—of the embryo apparently having sometimes a higher organisation +than the mature animal, into which it is developed. I believe that all +these facts can be explained, as follows, on the view of descent with +modification. + +It is commonly assumed, perhaps from monstrosities often affecting the +embryo at a very early period, that slight variations necessarily +appear at an equally early period. But we have little evidence on this +head—indeed the evidence rather points the other way; for it is +notorious that breeders of cattle, horses, and various fancy animals, +cannot positively tell, until some time after the animal has been born, +what its merits or form will ultimately turn out. We see this plainly +in our own children; we cannot always tell whether the child will be +tall or short, or what its precise features will be. The question is +not, at what period of life any variation has been caused, but at what +period it is fully displayed. The cause may have acted, and I believe +generally has acted, even before the embryo is formed; and the +variation may be due to the male and female sexual elements having been +affected by the conditions to which either parent, or their ancestors, +have been exposed. Nevertheless an effect thus caused at a very early +period, even before the formation of the embryo, may appear late in +life; as when an hereditary disease, which appears in old age alone, +has been communicated to the offspring from the reproductive element of +one parent. Or again, as when the horns of cross-bred cattle have been +affected by the shape of the horns of either parent. For the welfare of +a very young animal, as long as it remains in its mother’s womb, or in +the egg, or as long as it is nourished and protected by its parent, it +must be quite unimportant whether most of its characters are fully +acquired a little earlier or later in life. It would not signify, for +instance, to a bird which obtained its food best by having a long beak, +whether or not it assumed a beak of this particular length, as long as +it was fed by its parents. Hence, I conclude, that it is quite +possible, that each of the many successive modifications, by which each +species has acquired its present structure, may have supervened at a +not very early period of life; and some direct evidence from our +domestic animals supports this view. But in other cases it is quite +possible that each successive modification, or most of them, may have +appeared at an extremely early period. + +I have stated in the first chapter, that there is some evidence to +render it probable, that at whatever age any variation first appears in +the parent, it tends to reappear at a corresponding age in the +offspring. Certain variations can only appear at corresponding ages, +for instance, peculiarities in the caterpillar, cocoon, or imago states +of the silk-moth; or, again, in the horns of almost full-grown cattle. +But further than this, variations which, for all that we can see, might +have appeared earlier or later in life, tend to appear at a +corresponding age in the offspring and parent. I am far from meaning +that this is invariably the case; and I could give a good many cases of +variations (taking the word in the largest sense) which have supervened +at an earlier age in the child than in the parent. + +These two principles, if their truth be admitted, will, I believe, +explain all the above specified leading facts in embryology. But first +let us look at a few analogous cases in domestic varieties. Some +authors who have written on Dogs, maintain that the greyhound and +bulldog, though appearing so different, are really varieties most +closely allied, and have probably descended from +the same wild stock; hence I was curious to see how far their puppies +differed from each other: I was told by breeders that they differed +just as much as their parents, and this, judging by the eye, seemed +almost to be the case; but on actually measuring the old dogs and their +six-days old puppies, I found that the puppies had not nearly acquired +their full amount of proportional difference. So, again, I was told +that the foals of cart and race-horses differed as much as the +full-grown animals; and this surprised me greatly, as I think it +probable that the difference between these two breeds has been wholly +caused by selection under domestication; but having had careful +measurements made of the dam and of a three-days old colt of a race and +heavy cart-horse, I find that the colts have by no means acquired their +full amount of proportional difference. + +As the evidence appears to me conclusive, that the several domestic +breeds of Pigeon have descended from one wild species, I compared young +pigeons of various breeds, within twelve hours after being hatched; I +carefully measured the proportions (but will not here give details) of +the beak, width of mouth, length of nostril and of eyelid, size of feet +and length of leg, in the wild stock, in pouters, fantails, runts, +barbs, dragons, carriers, and tumblers. Now some of these birds, when +mature, differ so extraordinarily in length and form of beak, that they +would, I cannot doubt, be ranked in distinct genera, had they been +natural productions. But when the nestling birds of these several +breeds were placed in a row, though most of them could be distinguished +from each other, yet their proportional differences in the above +specified several points were incomparably less than in the full-grown +birds. Some characteristic points of difference—for instance, that of +the width of mouth—could hardly be detected in the young. +But there was one remarkable exception to this rule, for the young of +the short-faced tumbler differed from the young of the wild rock-pigeon +and of the other breeds, in all its proportions, almost exactly as much +as in the adult state. + +The two principles above given seem to me to explain these facts in +regard to the later embryonic stages of our domestic varieties. +Fanciers select their horses, dogs, and pigeons, for breeding, when +they are nearly grown up: they are indifferent whether the desired +qualities and structures have been acquired earlier or later in life, +if the full-grown animal possesses them. And the cases just given, more +especially that of pigeons, seem to show that the characteristic +differences which give value to each breed, and which have been +accumulated by man’s selection, have not generally first appeared at an +early period of life, and have been inherited by the offspring at a +corresponding not early period. But the case of the short-faced +tumbler, which when twelve hours old had acquired its proper +proportions, proves that this is not the universal rule; for here the +characteristic differences must either have appeared at an earlier +period than usual, or, if not so, the differences must have been +inherited, not at the corresponding, but at an earlier age. + +Now let us apply these facts and the above two principles—which latter, +though not proved true, can be shown to be in some degree probable—to +species in a state of nature. Let us take a genus of birds, descended +on my theory from some one parent-species, and of which the several new +species have become modified through natural selection in accordance +with their diverse habits. Then, from the many slight successive steps +of variation having supervened at a rather late age, and having been +inherited at a corresponding +age, the young of the new species of our supposed genus will manifestly +tend to resemble each other much more closely than do the adults, just +as we have seen in the case of pigeons. We may extend this view to +whole families or even classes. The fore-limbs, for instance, which +served as legs in the parent-species, may become, by a long course of +modification, adapted in one descendant to act as hands, in another as +paddles, in another as wings; and on the above two principles—namely of +each successive modification supervening at a rather late age, and +being inherited at a corresponding late age—the fore-limbs in the +embryos of the several descendants of the parent-species will still +resemble each other closely, for they will not have been modified. But +in each individual new species, the embryonic fore-limbs will differ +greatly from the fore-limbs in the mature animal; the limbs in the +latter having undergone much modification at a rather late period of +life, and having thus been converted into hands, or paddles, or wings. +Whatever influence long-continued exercise or use on the one hand, and +disuse on the other, may have in modifying an organ, such influence +will mainly affect the mature animal, which has come to its full powers +of activity and has to gain its own living; and the effects thus +produced will be inherited at a corresponding mature age. Whereas the +young will remain unmodified, or be modified in a lesser degree, by the +effects of use and disuse. + +In certain cases the successive steps of variation might supervene, +from causes of which we are wholly ignorant, at a very early period of +life, or each step might be inherited at an earlier period than that at +which it first appeared. In either case (as with the short-faced +tumbler) the young or embryo would closely +resemble the mature parent-form. We have seen that this is the rule of +development in certain whole groups of animals, as with cuttle-fish and +spiders, and with a few members of the great class of insects, as with +Aphis. With respect to the final cause of the young in these cases not +undergoing any metamorphosis, or closely resembling their parents from +their earliest age, we can see that this would result from the two +following contingencies; firstly, from the young, during a course of +modification carried on for many generations, having to provide for +their own wants at a very early stage of development, and secondly, +from their following exactly the same habits of life with their +parents; for in this case, it would be indispensable for the existence +of the species, that the child should be modified at a very early age +in the same manner with its parents, in accordance with their similar +habits. Some further explanation, however, of the embryo not undergoing +any metamorphosis is perhaps requisite. If, on the other hand, it +profited the young to follow habits of life in any degree different +from those of their parent, and consequently to be constructed in a +slightly different manner, then, on the principle of inheritance at +corresponding ages, the active young or larvæ might easily be rendered +by natural selection different to any conceivable extent from their +parents. Such differences might, also, become correlated with +successive stages of development; so that the larvæ, in the first +stage, might differ greatly from the larvæ in the second stage, as we +have seen to be the case with cirripedes. The adult might become fitted +for sites or habits, in which organs of locomotion or of the senses, +etc., would be useless; and in this case the final metamorphosis would +be said to be retrograde. + +As all the organic beings, extinct and recent, which +have ever lived on this earth have to be classed together, and as all +have been connected by the finest gradations, the best, or indeed, if +our collections were nearly perfect, the only possible arrangement, +would be genealogical. Descent being on my view the hidden bond of +connexion which naturalists have been seeking under the term of the +natural system. On this view we can understand how it is that, in the +eyes of most naturalists, the structure of the embryo is even more +important for classification than that of the adult. For the embryo is +the animal in its less modified state; and in so far it reveals the +structure of its progenitor. In two groups of animal, however much they +may at present differ from each other in structure and habits, if they +pass through the same or similar embryonic stages, we may feel assured +that they have both descended from the same or nearly similar parents, +and are therefore in that degree closely related. Thus, community in +embryonic structure reveals community of descent. It will reveal this +community of descent, however much the structure of the adult may have +been modified and obscured; we have seen, for instance, that cirripedes +can at once be recognised by their larvæ as belonging to the great +class of crustaceans. As the embryonic state of each species and group +of species partially shows us the structure of their less modified +ancient progenitors, we can clearly see why ancient and extinct forms +of life should resemble the embryos of their descendants,—our existing +species. Agassiz believes this to be a law of nature; but I am bound to +confess that I only hope to see the law hereafter proved true. It can +be proved true in those cases alone in which the ancient state, now +supposed to be represented in many embryos, has not been obliterated, +either by the successive variations in a long course of modification +having supervened +at a very early age, or by the variations having been inherited at an +earlier period than that at which they first appeared. It should also +be borne in mind, that the supposed law of resemblance of ancient forms +of life to the embryonic stages of recent forms, may be true, but yet, +owing to the geological record not extending far enough back in time, +may remain for a long period, or for ever, incapable of demonstration. + +Thus, as it seems to me, the leading facts in embryology, which are +second in importance to none in natural history, are explained on the +principle of slight modifications not appearing, in the many +descendants from some one ancient progenitor, at a very early period in +the life of each, though perhaps caused at the earliest, and being +inherited at a corresponding not early period. Embryology rises greatly +in interest, when we thus look at the embryo as a picture, more or less +obscured, of the common parent-form of each great class of animals. + +_Rudimentary, atrophied, or aborted organs_.—Organs or parts in this +strange condition, bearing the stamp of inutility, are extremely common +throughout nature. For instance, rudimentary mammæ are very general in +the males of mammals: I presume that the “bastard-wing” in birds may be +safely considered as a digit in a rudimentary state: in very many +snakes one lobe of the lungs is rudimentary; in other snakes there are +rudiments of the pelvis and hind limbs. Some of the cases of +rudimentary organs are extremely curious; for instance, the presence of +teeth in foetal whales, which when grown up have not a tooth in their +heads; and the presence of teeth, which never cut through the gums, in +the upper jaws of our unborn calves. It has even been stated on good +authority that rudiments of teeth can be detected +in the beaks of certain embryonic birds. Nothing can be plainer than +that wings are formed for flight, yet in how many insects do we see +wings so reduced in size as to be utterly incapable of flight, and not +rarely lying under wing-cases, firmly soldered together! + +The meaning of rudimentary organs is often quite unmistakeable: for +instance there are beetles of the same genus (and even of the same +species) resembling each other most closely in all respects, one of +which will have full-sized wings, and another mere rudiments of +membrane; and here it is impossible to doubt, that the rudiments +represent wings. Rudimentary organs sometimes retain their +potentiality, and are merely not developed: this seems to be the case +with the mammæ of male mammals, for many instances are on record of +these organs having become well developed in full-grown males, and +having secreted milk. So again there are normally four developed and +two rudimentary teats in the udders of the genus Bos, but in our +domestic cows the two sometimes become developed and give milk. In +individual plants of the same species the petals sometimes occur as +mere rudiments, and sometimes in a well-developed state. In plants with +separated sexes, the male flowers often have a rudiment of a pistil; +and Kölreuter found that by crossing such male plants with an +hermaphrodite species, the rudiment of the pistil in the hybrid +offspring was much increased in size; and this shows that the rudiment +and the perfect pistil are essentially alike in nature. + +An organ serving for two purposes, may become rudimentary or utterly +aborted for one, even the more important purpose; and remain perfectly +efficient for the other. Thus in plants, the office of the pistil is to +allow the pollen-tubes to reach the ovules protected in the ovarium at +its base. The pistil consists of a stigma +supported on the style; but in some Compositæ, the male florets, which +of course cannot be fecundated, have a pistil, which is in a +rudimentary state, for it is not crowned with a stigma; but the style +remains well developed, and is clothed with hairs as in other +compositæ, for the purpose of brushing the pollen out of the +surrounding anthers. Again, an organ may become rudimentary for its +proper purpose, and be used for a distinct object: in certain fish the +swim-bladder seems to be rudimentary for its proper function of giving +buoyancy, but has become converted into a nascent breathing organ or +lung. Other similar instances could be given. + +Rudimentary organs in the individuals of the same species are very +liable to vary in degree of development and in other respects. +Moreover, in closely allied species, the degree to which the same organ +has been rendered rudimentary occasionally differs much. This latter +fact is well exemplified in the state of the wings of the female moths +in certain groups. Rudimentary organs may be utterly aborted; and this +implies, that we find in an animal or plant no trace of an organ, which +analogy would lead us to expect to find, and which is occasionally +found in monstrous individuals of the species. Thus in the snapdragon +(antirrhinum) we generally do not find a rudiment of a fifth stamen; +but this may sometimes be seen. In tracing the homologies of the same +part in different members of a class, nothing is more common, or more +necessary, than the use and discovery of rudiments. This is well shown +in the drawings given by Owen of the bones of the leg of the horse, ox, +and rhinoceros. + +It is an important fact that rudimentary organs, such as teeth in the +upper jaws of whales and ruminants, can often be detected in the +embryo, but afterwards wholly disappear. It is also, I believe, a +universal +rule, that a rudimentary part or organ is of greater size relatively to +the adjoining parts in the embryo, than in the adult; so that the organ +at this early age is less rudimentary, or even cannot be said to be in +any degree rudimentary. Hence, also, a rudimentary organ in the adult, +is often said to have retained its embryonic condition. + +I have now given the leading facts with respect to rudimentary organs. +In reflecting on them, every one must be struck with astonishment: for +the same reasoning power which tells us plainly that most parts and +organs are exquisitely adapted for certain purposes, tells us with +equal plainness that these rudimentary or atrophied organs, are +imperfect and useless. In works on natural history rudimentary organs +are generally said to have been created “for the sake of symmetry,” or +in order “to complete the scheme of nature;” but this seems to me no +explanation, merely a restatement of the fact. Would it be thought +sufficient to say that because planets revolve in elliptic courses +round the sun, satellites follow the same course round the planets, for +the sake of symmetry, and to complete the scheme of nature? An eminent +physiologist accounts for the presence of rudimentary organs, by +supposing that they serve to excrete matter in excess, or injurious to +the system; but can we suppose that the minute papilla, which often +represents the pistil in male flowers, and which is formed merely of +cellular tissue, can thus act? Can we suppose that the formation of +rudimentary teeth which are subsequently absorbed, can be of any +service to the rapidly growing embryonic calf by the excretion of +precious phosphate of lime? When a man’s fingers have been amputated, +imperfect nails sometimes appear on the stumps: I could as soon believe +that these vestiges of nails have appeared, not from unknown laws +of growth, but in order to excrete horny matter, as that the +rudimentary nails on the fin of the manatee were formed for this +purpose. + +On my view of descent with modification, the origin of rudimentary +organs is simple. We have plenty of cases of rudimentary organs in our +domestic productions,—as the stump of a tail in tailless breeds,—the +vestige of an ear in earless breeds,—the reappearance of minute +dangling horns in hornless breeds of cattle, more especially, according +to Youatt, in young animals,—and the state of the whole flower in the +cauliflower. We often see rudiments of various parts in monsters. But I +doubt whether any of these cases throw light on the origin of +rudimentary organs in a state of nature, further than by showing that +rudiments can be produced; for I doubt whether species under nature +ever undergo abrupt changes. I believe that disuse has been the main +agency; that it has led in successive generations to the gradual +reduction of various organs, until they have become rudimentary,—as in +the case of the eyes of animals inhabiting dark caverns, and of the +wings of birds inhabiting oceanic islands, which have seldom been +forced to take flight, and have ultimately lost the power of flying. +Again, an organ useful under certain conditions, might become injurious +under others, as with the wings of beetles living on small and exposed +islands; and in this case natural selection would continue slowly to +reduce the organ, until it was rendered harmless and rudimentary. + +Any change in function, which can be effected by insensibly small +steps, is within the power of natural selection; so that an organ +rendered, during changed habits of life, useless or injurious for one +purpose, might easily be modified and used for another purpose. Or an +organ might be retained for one alone of its +former functions. An organ, when rendered useless, may well be +variable, for its variations cannot be checked by natural selection. At +whatever period of life disuse or selection reduces an organ, and this +will generally be when the being has come to maturity and to its full +powers of action, the principle of inheritance at corresponding ages +will reproduce the organ in its reduced state at the same age, and +consequently will seldom affect or reduce it in the embryo. Thus we can +understand the greater relative size of rudimentary organs in the +embryo, and their lesser relative size in the adult. But if each step +of the process of reduction were to be inherited, not at the +corresponding age, but at an extremely early period of life (as we have +good reason to believe to be possible) the rudimentary part would tend +to be wholly lost, and we should have a case of complete abortion. The +principle, also, of economy, explained in a former chapter, by which +the materials forming any part or structure, if not useful to the +possessor, will be saved as far as is possible, will probably often +come into play; and this will tend to cause the entire obliteration of +a rudimentary organ. + +As the presence of rudimentary organs is thus due to the tendency in +every part of the organisation, which has long existed, to be +inherited—we can understand, on the genealogical view of +classification, how it is that systematists have found rudimentary +parts as useful as, or even sometimes more useful than, parts of high +physiological importance. Rudimentary organs may be compared with the +letters in a word, still retained in the spelling, but become useless +in the pronunciation, but which serve as a clue in seeking for its +derivation. On the view of descent with modification, we may conclude +that the existence of organs in a rudimentary, imperfect, and useless +condition, or quite aborted, far +from presenting a strange difficulty, as they assuredly do on the +ordinary doctrine of creation, might even have been anticipated, and +can be accounted for by the laws of inheritance. + +_Summary_.—In this chapter I have attempted to show, that the +subordination of group to group in all organisms throughout all time; +that the nature of the relationship, by which all living and extinct +beings are united by complex, radiating, and circuitous lines of +affinities into one grand system; the rules followed and the +difficulties encountered by naturalists in their classifications; the +value set upon characters, if constant and prevalent, whether of high +vital importance, or of the most trifling importance, or, as in +rudimentary organs, of no importance; the wide opposition in value +between analogical or adaptive characters, and characters of true +affinity; and other such rules;—all naturally follow on the view of the +common parentage of those forms which are considered by naturalists as +allied, together with their modification through natural selection, +with its contingencies of extinction and divergence of character. In +considering this view of classification, it should be borne in mind +that the element of descent has been universally used in ranking +together the sexes, ages, and acknowledged varieties of the same +species, however different they may be in structure. If we extend the +use of this element of descent,—the only certainly known cause of +similarity in organic beings,—we shall understand what is meant by the +natural system: it is genealogical in its attempted arrangement, with +the grades of acquired difference marked by the terms varieties, +species, genera, families, orders, and classes. + +On this same view of descent with modification, all the great facts in +Morphology become intelligible,—whether +we look to the same pattern displayed in the homologous organs, to +whatever purpose applied, of the different species of a class; or to +the homologous parts constructed on the same pattern in each individual +animal and plant. + +On the principle of successive slight variations, not necessarily or +generally supervening at a very early period of life, and being +inherited at a corresponding period, we can understand the great +leading facts in Embryology; namely, the resemblance in an individual +embryo of the homologous parts, which when matured will become widely +different from each other in structure and function; and the +resemblance in different species of a class of the homologous parts or +organs, though fitted in the adult members for purposes as different as +possible. Larvæ are active embryos, which have become specially +modified in relation to their habits of life, through the principle of +modifications being inherited at corresponding ages. On this same +principle—and bearing in mind, that when organs are reduced in size, +either from disuse or selection, it will generally be at that period of +life when the being has to provide for its own wants, and bearing in +mind how strong is the principle of inheritance—the occurrence of +rudimentary organs and their final abortion, present to us no +inexplicable difficulties; on the contrary, their presence might have +been even anticipated. The importance of embryological characters and +of rudimentary organs in classification is intelligible, on the view +that an arrangement is only so far natural as it is genealogical. + +Finally, the several classes of facts which have been considered in +this chapter, seem to me to proclaim so plainly, that the innumerable +species, genera, and families of organic beings, with which this world +is +peopled, have all descended, each within its own class or group, from +common parents, and have all been modified in the course of descent, +that I should without hesitation adopt this view, even if it were +unsupported by other facts or arguments. + + + + +CHAPTER XIV. +RECAPITULATION AND CONCLUSION. + + +Recapitulation of the difficulties on the theory of Natural Selection. +Recapitulation of the general and special circumstances in its favour. +Causes of the general belief in the immutability of species. How far +the theory of natural selection may be extended. Effects of its +adoption on the study of Natural history. Concluding remarks. + + +As this whole volume is one long argument, it may be convenient to the +reader to have the leading facts and inferences briefly recapitulated. + +That many and grave objections may be advanced against the theory of +descent with modification through natural selection, I do not deny. I +have endeavoured to give to them their full force. Nothing at first can +appear more difficult to believe than that the more complex organs and +instincts should have been perfected, not by means superior to, though +analogous with, human reason, but by the accumulation of innumerable +slight variations, each good for the individual possessor. +Nevertheless, this difficulty, though appearing to our imagination +insuperably great, cannot be considered real if we admit the following +propositions, namely,—that gradations in the perfection of any organ or +instinct, which we may consider, either do now exist or could have +existed, each good of its kind,—that all organs and instincts are, in +ever so slight a degree, variable,—and, lastly, that there is a +struggle for existence leading to the preservation of each profitable +deviation of structure or instinct. The truth of these propositions +cannot, I think, be disputed. + + +It is, no doubt, extremely difficult even to conjecture by what +gradations many structures have been perfected, more especially amongst +broken and failing groups of organic beings; but we see so many strange +gradations in nature, as is proclaimed by the canon, “Natura non facit +saltum,” that we ought to be extremely cautious in saying that any +organ or instinct, or any whole being, could not have arrived at its +present state by many graduated steps. There are, it must be admitted, +cases of special difficulty on the theory of natural selection; and one +of the most curious of these is the existence of two or three defined +castes of workers or sterile females in the same community of ants; but +I have attempted to show how this difficulty can be mastered. + +With respect to the almost universal sterility of species when first +crossed, which forms so remarkable a contrast with the almost universal +fertility of varieties when crossed, I must refer the reader to the +recapitulation of the facts given at the end of the eighth chapter, +which seem to me conclusively to show that this sterility is no more a +special endowment than is the incapacity of two trees to be grafted +together, but that it is incidental on constitutional differences in +the reproductive systems of the intercrossed species. We see the truth +of this conclusion in the vast difference in the result, when the same +two species are crossed reciprocally; that is, when one species is +first used as the father and then as the mother. + +The fertility of varieties when intercrossed and of their mongrel +offspring cannot be considered as universal; nor is their very general +fertility surprising when we remember that it is not likely that either +their constitutions or their reproductive systems should have been +profoundly modified. Moreover, most of the +varieties which have been experimentised on have been produced under +domestication; and as domestication apparently tends to eliminate +sterility, we ought not to expect it also to produce sterility. + +The sterility of hybrids is a very different case from that of first +crosses, for their reproductive organs are more or less functionally +impotent; whereas in first crosses the organs on both sides are in a +perfect condition. As we continually see that organisms of all kinds +are rendered in some degree sterile from their constitutions having +been disturbed by slightly different and new conditions of life, we +need not feel surprise at hybrids being in some degree sterile, for +their constitutions can hardly fail to have been disturbed from being +compounded of two distinct organisations. This parallelism is supported +by another parallel, but directly opposite, class of facts; namely, +that the vigour and fertility of all organic beings are increased by +slight changes in their conditions of life, and that the offspring of +slightly modified forms or varieties acquire from being crossed +increased vigour and fertility. So that, on the one hand, considerable +changes in the conditions of life and crosses between greatly modified +forms, lessen fertility; and on the other hand, lesser changes in the +conditions of life and crosses between less modified forms, increase +fertility. + +Turning to geographical distribution, the difficulties encountered on +the theory of descent with modification are grave enough. All the +individuals of the same species, and all the species of the same genus, +or even higher group, must have descended from common parents; and +therefore, in however distant and isolated parts of the world they are +now found, they must in the course of successive generations have +passed from some one part to the others. We are often wholly unable +even to conjecture how this could have been effected. Yet, as we have +reason to believe that some species have retained the same specific +form for very long periods, enormously long as measured by years, too +much stress ought not to be laid on the occasional wide diffusion of +the same species; for during very long periods of time there will +always be a good chance for wide migration by many means. A broken or +interrupted range may often be accounted for by the extinction of the +species in the intermediate regions. It cannot be denied that we are as +yet very ignorant of the full extent of the various climatal and +geographical changes which have affected the earth during modern +periods; and such changes will obviously have greatly facilitated +migration. As an example, I have attempted to show how potent has been +the influence of the Glacial period on the distribution both of the +same and of representative species throughout the world. We are as yet +profoundly ignorant of the many occasional means of transport. With +respect to distinct species of the same genus inhabiting very distant +and isolated regions, as the process of modification has necessarily +been slow, all the means of migration will have been possible during a +very long period; and consequently the difficulty of the wide diffusion +of species of the same genus is in some degree lessened. + +As on the theory of natural selection an interminable number of +intermediate forms must have existed, linking together all the species +in each group by gradations as fine as our present varieties, it may be +asked, Why do we not see these linking forms all around us? Why are not +all organic beings blended together in an inextricable chaos? With +respect to existing forms, we should remember that we have no right to +expect (excepting in rare cases) to discover _directly_ connecting +links between them, but only between each and some extinct and +supplanted form. Even on a wide area, which has during a long period +remained continuous, and of which the climate and other conditions of +life change insensibly in going from a district occupied by one species +into another district occupied by a closely allied species, we have no +just right to expect often to find intermediate varieties in the +intermediate zone. For we have reason to believe that only a few +species are undergoing change at any one period; and all changes are +slowly effected. I have also shown that the intermediate varieties +which will at first probably exist in the intermediate zones, will be +liable to be supplanted by the allied forms on either hand; and the +latter, from existing in greater numbers, will generally be modified +and improved at a quicker rate than the intermediate varieties, which +exist in lesser numbers; so that the intermediate varieties will, in +the long run, be supplanted and exterminated. + +On this doctrine of the extermination of an infinitude of connecting +links, between the living and extinct inhabitants of the world, and at +each successive period between the extinct and still older species, why +is not every geological formation charged with such links? Why does not +every collection of fossil remains afford plain evidence of the +gradation and mutation of the forms of life? We meet with no such +evidence, and this is the most obvious and forcible of the many +objections which may be urged against my theory. Why, again, do whole +groups of allied species appear, though certainly they often falsely +appear, to have come in suddenly on the several geological stages? Why +do we not find great piles of strata beneath the Silurian system, +stored with the remains of the progenitors of the Silurian groups of +fossils? For certainly on my theory such +strata must somewhere have been deposited at these ancient and utterly +unknown epochs in the world’s history. + +I can answer these questions and grave objections only on the +supposition that the geological record is far more imperfect than most +geologists believe. It cannot be objected that there has not been time +sufficient for any amount of organic change; for the lapse of time has +been so great as to be utterly inappreciable by the human intellect. +The number of specimens in all our museums is absolutely as nothing +compared with the countless generations of countless species which +certainly have existed. We should not be able to recognise a species as +the parent of any one or more species if we were to examine them ever +so closely, unless we likewise possessed many of the intermediate links +between their past or parent and present states; and these many links +we could hardly ever expect to discover, owing to the imperfection of +the geological record. Numerous existing doubtful forms could be named +which are probably varieties; but who will pretend that in future ages +so many fossil links will be discovered, that naturalists will be able +to decide, on the common view, whether or not these doubtful forms are +varieties? As long as most of the links between any two species are +unknown, if any one link or intermediate variety be discovered, it will +simply be classed as another and distinct species. Only a small portion +of the world has been geologically explored. Only organic beings of +certain classes can be preserved in a fossil condition, at least in any +great number. Widely ranging species vary most, and varieties are often +at first local,—both causes rendering the discovery of intermediate +links less likely. Local varieties will not spread into other and +distant regions until they are considerably modified and improved; +and when they do spread, if discovered in a geological formation, they +will appear as if suddenly created there, and will be simply classed as +new species. Most formations have been intermittent in their +accumulation; and their duration, I am inclined to believe, has been +shorter than the average duration of specific forms. Successive +formations are separated from each other by enormous blank intervals of +time; for fossiliferous formations, thick enough to resist future +degradation, can be accumulated only where much sediment is deposited +on the subsiding bed of the sea. During the alternate periods of +elevation and of stationary level the record will be blank. During +these latter periods there will probably be more variability in the +forms of life; during periods of subsidence, more extinction. + +With respect to the absence of fossiliferous formations beneath the +lowest Silurian strata, I can only recur to the hypothesis given in the +ninth chapter. That the geological record is imperfect all will admit; +but that it is imperfect to the degree which I require, few will be +inclined to admit. If we look to long enough intervals of time, geology +plainly declares that all species have changed; and they have changed +in the manner which my theory requires, for they have changed slowly +and in a graduated manner. We clearly see this in the fossil remains +from consecutive formations invariably being much more closely related +to each other, than are the fossils from formations distant from each +other in time. + +Such is the sum of the several chief objections and difficulties which +may justly be urged against my theory; and I have now briefly +recapitulated the answers and explanations which can be given to them. +I have felt these difficulties far too heavily during many years to +doubt their weight. But it deserves especial notice that the more +important objections relate to questions on which we are confessedly +ignorant; nor do we know how ignorant we are. We do not know all the +possible transitional gradations between the simplest and the most +perfect organs; it cannot be pretended that we know all the varied +means of Distribution during the long lapse of years, or that we know +how imperfect the Geological Record is. Grave as these several +difficulties are, in my judgment they do not overthrow the theory of +descent with modification. + +Now let us turn to the other side of the argument. Under domestication +we see much variability. This seems to be mainly due to the +reproductive system being eminently susceptible to changes in the +conditions of life; so that this system, when not rendered impotent, +fails to reproduce offspring exactly like the parent-form. Variability +is governed by many complex laws,—by correlation of growth, by use and +disuse, and by the direct action of the physical conditions of life. +There is much difficulty in ascertaining how much modification our +domestic productions have undergone; but we may safely infer that the +amount has been large, and that modifications can be inherited for long +periods. As long as the conditions of life remain the same, we have +reason to believe that a modification, which has already been inherited +for many generations, may continue to be inherited for an almost +infinite number of generations. On the other hand we have evidence that +variability, when it has once come into play, does not wholly cease; +for new varieties are still occasionally produced by our most anciently +domesticated productions. + +Man does not actually produce variability; he only +unintentionally exposes organic beings to new conditions of life, and +then nature acts on the organisation, and causes variability. But man +can and does select the variations given to him by nature, and thus +accumulate them in any desired manner. He thus adapts animals and +plants for his own benefit or pleasure. He may do this methodically, or +he may do it unconsciously by preserving the individuals most useful to +him at the time, without any thought of altering the breed. It is +certain that he can largely influence the character of a breed by +selecting, in each successive generation, individual differences so +slight as to be quite inappreciable by an uneducated eye. This process +of selection has been the great agency in the production of the most +distinct and useful domestic breeds. That many of the breeds produced +by man have to a large extent the character of natural species, is +shown by the inextricable doubts whether very many of them are +varieties or aboriginal species. + +There is no obvious reason why the principles which have acted so +efficiently under domestication should not have acted under nature. In +the preservation of favoured individuals and races, during the +constantly-recurrent Struggle for Existence, we see the most powerful +and ever-acting means of selection. The struggle for existence +inevitably follows from the high geometrical ratio of increase which is +common to all organic beings. This high rate of increase is proved by +calculation, by the effects of a succession of peculiar seasons, and by +the results of naturalisation, as explained in the third chapter. More +individuals are born than can possibly survive. A grain in the balance +will determine which individual shall live and which shall die,—which +variety or species shall increase in number, and which shall decrease, +or finally become extinct. As the individuals +of the same species come in all respects into the closest competition +with each other, the struggle will generally be most severe between +them; it will be almost equally severe between the varieties of the +same species, and next in severity between the species of the same +genus. But the struggle will often be very severe between beings most +remote in the scale of nature. The slightest advantage in one being, at +any age or during any season, over those with which it comes into +competition, or better adaptation in however slight a degree to the +surrounding physical conditions, will turn the balance. + +With animals having separated sexes there will in most cases be a +struggle between the males for possession of the females. The most +vigorous individuals, or those which have most successfully struggled +with their conditions of life, will generally leave most progeny. But +success will often depend on having special weapons or means of +defence, or on the charms of the males; and the slightest advantage +will lead to victory. + +As geology plainly proclaims that each land has undergone great +physical changes, we might have expected that organic beings would have +varied under nature, in the same way as they generally have varied +under the changed conditions of domestication. And if there be any +variability under nature, it would be an unaccountable fact if natural +selection had not come into play. It has often been asserted, but the +assertion is quite incapable of proof, that the amount of variation +under nature is a strictly limited quantity. Man, though acting on +external characters alone and often capriciously, can produce within a +short period a great result by adding up mere individual differences in +his domestic productions; and every one admits that there are at least +individual differences in species under nature. But, besides such +differences, all naturalists +have admitted the existence of varieties, which they think sufficiently +distinct to be worthy of record in systematic works. No one can draw +any clear distinction between individual differences and slight +varieties; or between more plainly marked varieties and sub-species, +and species. Let it be observed how naturalists differ in the rank +which they assign to the many representative forms in Europe and North +America. + +If then we have under nature variability and a powerful agent always +ready to act and select, why should we doubt that variations in any way +useful to beings, under their excessively complex relations of life, +would be preserved, accumulated, and inherited? Why, if man can by +patience select variations most useful to himself, should nature fail +in selecting variations useful, under changing conditions of life, to +her living products? What limit can be put to this power, acting during +long ages and rigidly scrutinising the whole constitution, structure, +and habits of each creature,—favouring the good and rejecting the bad? +I can see no limit to this power, in slowly and beautifully adapting +each form to the most complex relations of life. The theory of natural +selection, even if we looked no further than this, seems to me to be in +itself probable. I have already recapitulated, as fairly as I could, +the opposed difficulties and objections: now let us turn to the special +facts and arguments in favour of the theory. + +On the view that species are only strongly marked and permanent +varieties, and that each species first existed as a variety, we can see +why it is that no line of demarcation can be drawn between species, +commonly supposed to have been produced by special acts of creation, +and varieties which are acknowledged to have been produced by secondary +laws. On this same view we can understand how it is that in each region +where many species of a genus have been produced, and where they now +flourish, these same species should present many varieties; for where +the manufactory of species has been active, we might expect, as a +general rule, to find it still in action; and this is the case if +varieties be incipient species. Moreover, the species of the larger +genera, which afford the greater number of varieties or incipient +species, retain to a certain degree the character of varieties; for +they differ from each other by a less amount of difference than do the +species of smaller genera. The closely allied species also of the +larger genera apparently have restricted ranges, and they are clustered +in little groups round other species—in which respects they resemble +varieties. These are strange relations on the view of each species +having been independently created, but are intelligible if all species +first existed as varieties. + +As each species tends by its geometrical ratio of reproduction to +increase inordinately in number; and as the modified descendants of +each species will be enabled to increase by so much the more as they +become more diversified in habits and structure, so as to be enabled to +seize on many and widely different places in the economy of nature, +there will be a constant tendency in natural selection to preserve the +most divergent offspring of any one species. Hence during a +long-continued course of modification, the slight differences, +characteristic of varieties of the same species, tend to be augmented +into the greater differences characteristic of species of the same +genus. New and improved varieties will inevitably supplant and +exterminate the older, less improved and intermediate varieties; and +thus species are rendered to a large extent defined and distinct +objects. Dominant species belonging to the larger groups tend to give +birth to new and dominant +forms; so that each large group tends to become still larger, and at +the same time more divergent in character. But as all groups cannot +thus succeed in increasing in size, for the world would not hold them, +the more dominant groups beat the less dominant. This tendency in the +large groups to go on increasing in size and diverging in character, +together with the almost inevitable contingency of much extinction, +explains the arrangement of all the forms of life, in groups +subordinate to groups, all within a few great classes, which we now see +everywhere around us, and which has prevailed throughout all time. This +grand fact of the grouping of all organic beings seems to me utterly +inexplicable on the theory of creation. + +As natural selection acts solely by accumulating slight, successive, +favourable variations, it can produce no great or sudden modification; +it can act only by very short and slow steps. Hence the canon of +“Natura non facit saltum,” which every fresh addition to our knowledge +tends to make more strictly correct, is on this theory simply +intelligible. We can plainly see why nature is prodigal in variety, +though niggard in innovation. But why this should be a law of nature if +each species has been independently created, no man can explain. + +Many other facts are, as it seems to me, explicable on this theory. How +strange it is that a bird, under the form of woodpecker, should have +been created to prey on insects on the ground; that upland geese, which +never or rarely swim, should have been created with webbed feet; that a +thrush should have been created to dive and feed on sub-aquatic +insects; and that a petrel should have been created with habits and +structure fitting it for the life of an auk or grebe! and so on in +endless other cases. But on the view of each +species constantly trying to increase in number, with natural selection +always ready to adapt the slowly varying descendants of each to any +unoccupied or ill-occupied place in nature, these facts cease to be +strange, or perhaps might even have been anticipated. + +As natural selection acts by competition, it adapts the inhabitants of +each country only in relation to the degree of perfection of their +associates; so that we need feel no surprise at the inhabitants of any +one country, although on the ordinary view supposed to have been +specially created and adapted for that country, being beaten and +supplanted by the naturalised productions from another land. Nor ought +we to marvel if all the contrivances in nature be not, as far as we can +judge, absolutely perfect; and if some of them be abhorrent to our +ideas of fitness. We need not marvel at the sting of the bee causing +the bee’s own death; at drones being produced in such vast numbers for +one single act, and being then slaughtered by their sterile sisters; at +the astonishing waste of pollen by our fir-trees; at the instinctive +hatred of the queen bee for her own fertile daughters; at ichneumonidæ +feeding within the live bodies of caterpillars; and at other such +cases. The wonder indeed is, on the theory of natural selection, that +more cases of the want of absolute perfection have not been observed. + +The complex and little known laws governing variation are the same, as +far as we can see, with the laws which have governed the production of +so-called specific forms. In both cases physical conditions seem to +have produced but little direct effect; yet when varieties enter any +zone, they occasionally assume some of the characters of the species +proper to that zone. In both varieties and species, use and disuse seem +to have produced some effect; for it is difficult to resist this +conclusion +when we look, for instance, at the logger-headed duck, which has wings +incapable of flight, in nearly the same condition as in the domestic +duck; or when we look at the burrowing tucutucu, which is occasionally +blind, and then at certain moles, which are habitually blind and have +their eyes covered with skin; or when we look at the blind animals +inhabiting the dark caves of America and Europe. In both varieties and +species correlation of growth seems to have played a most important +part, so that when one part has been modified other parts are +necessarily modified. In both varieties and species reversions to +long-lost characters occur. How inexplicable on the theory of creation +is the occasional appearance of stripes on the shoulder and legs of the +several species of the horse-genus and in their hybrids! How simply is +this fact explained if we believe that these species have descended +from a striped progenitor, in the same manner as the several domestic +breeds of pigeon have descended from the blue and barred rock-pigeon! + +On the ordinary view of each species having been independently created, +why should the specific characters, or those by which the species of +the same genus differ from each other, be more variable than the +generic characters in which they all agree? Why, for instance, should +the colour of a flower be more likely to vary in any one species of a +genus, if the other species, supposed to have been created +independently, have differently coloured flowers, than if all the +species of the genus have the same coloured flowers? If species are +only well-marked varieties, of which the characters have become in a +high degree permanent, we can understand this fact; for they have +already varied since they branched off from a common progenitor in +certain characters, by which they have come to be specifically distinct +from each other; +and therefore these same characters would be more likely still to be +variable than the generic characters which have been inherited without +change for an enormous period. It is inexplicable on the theory of +creation why a part developed in a very unusual manner in any one +species of a genus, and therefore, as we may naturally infer, of great +importance to the species, should be eminently liable to variation; +but, on my view, this part has undergone, since the several species +branched off from a common progenitor, an unusual amount of variability +and modification, and therefore we might expect this part generally to +be still variable. But a part may be developed in the most unusual +manner, like the wing of a bat, and yet not be more variable than any +other structure, if the part be common to many subordinate forms, that +is, if it has been inherited for a very long period; for in this case +it will have been rendered constant by long-continued natural +selection. + +Glancing at instincts, marvellous as some are, they offer no greater +difficulty than does corporeal structure on the theory of the natural +selection of successive, slight, but profitable modifications. We can +thus understand why nature moves by graduated steps in endowing +different animals of the same class with their several instincts. I +have attempted to show how much light the principle of gradation throws +on the admirable architectural powers of the hive-bee. Habit no doubt +sometimes comes into play in modifying instincts; but it certainly is +not indispensable, as we see, in the case of neuter insects, which +leave no progeny to inherit the effects of long-continued habit. On the +view of all the species of the same genus having descended from a +common parent, and having inherited much in common, we can understand +how it is that allied species, when placed under considerably different +conditions of life, +yet should follow nearly the same instincts; why the thrush of South +America, for instance, lines her nest with mud like our British +species. On the view of instincts having been slowly acquired through +natural selection we need not marvel at some instincts being apparently +not perfect and liable to mistakes, and at many instincts causing other +animals to suffer. + +If species be only well-marked and permanent varieties, we can at once +see why their crossed offspring should follow the same complex laws in +their degrees and kinds of resemblance to their parents,—in being +absorbed into each other by successive crosses, and in other such +points,—as do the crossed offspring of acknowledged varieties. On the +other hand, these would be strange facts if species have been +independently created, and varieties have been produced by secondary +laws. + +If we admit that the geological record is imperfect in an extreme +degree, then such facts as the record gives, support the theory of +descent with modification. New species have come on the stage slowly +and at successive intervals; and the amount of change, after equal +intervals of time, is widely different in different groups. The +extinction of species and of whole groups of species, which has played +so conspicuous a part in the history of the organic world, almost +inevitably follows on the principle of natural selection; for old forms +will be supplanted by new and improved forms. Neither single species +nor groups of species reappear when the chain of ordinary generation +has once been broken. The gradual diffusion of dominant forms, with the +slow modification of their descendants, causes the forms of life, after +long intervals of time, to appear as if they had changed simultaneously +throughout the world. The fact of the fossil remains of each formation +being in some degree intermediate in character between the +fossils in the formations above and below, is simply explained by their +intermediate position in the chain of descent. The grand fact that all +extinct organic beings belong to the same system with recent beings, +falling either into the same or into intermediate groups, follows from +the living and the extinct being the offspring of common parents. As +the groups which have descended from an ancient progenitor have +generally diverged in character, the progenitor with its early +descendants will often be intermediate in character in comparison with +its later descendants; and thus we can see why the more ancient a +fossil is, the oftener it stands in some degree intermediate between +existing and allied groups. Recent forms are generally looked at as +being, in some vague sense, higher than ancient and extinct forms; and +they are in so far higher as the later and more improved forms have +conquered the older and less improved organic beings in the struggle +for life. Lastly, the law of the long endurance of allied forms on the +same continent,—of marsupials in Australia, of edentata in America, and +other such cases,—is intelligible, for within a confined country, the +recent and the extinct will naturally be allied by descent. + +Looking to geographical distribution, if we admit that there has been +during the long course of ages much migration from one part of the +world to another, owing to former climatal and geographical changes and +to the many occasional and unknown means of dispersal, then we can +understand, on the theory of descent with modification, most of the +great leading facts in Distribution. We can see why there should be so +striking a parallelism in the distribution of organic beings throughout +space, and in their geological succession throughout time; for in both +cases the beings have been connected by the bond of ordinary +generation, and the means of +modification have been the same. We see the full meaning of the +wonderful fact, which must have struck every traveller, namely, that on +the same continent, under the most diverse conditions, under heat and +cold, on mountain and lowland, on deserts and marshes, most of the +inhabitants within each great class are plainly related; for they will +generally be descendants of the same progenitors and early colonists. +On this same principle of former migration, combined in most cases with +modification, we can understand, by the aid of the Glacial period, the +identity of some few plants, and the close alliance of many others, on +the most distant mountains, under the most different climates; and +likewise the close alliance of some of the inhabitants of the sea in +the northern and southern temperate zones, though separated by the +whole intertropical ocean. Although two areas may present the same +physical conditions of life, we need feel no surprise at their +inhabitants being widely different, if they have been for a long period +completely separated from each other; for as the relation of organism +to organism is the most important of all relations, and as the two +areas will have received colonists from some third source or from each +other, at various periods and in different proportions, the course of +modification in the two areas will inevitably be different. + +On this view of migration, with subsequent modification, we can see why +oceanic islands should be inhabited by few species, but of these, that +many should be peculiar. We can clearly see why those animals which +cannot cross wide spaces of ocean, as frogs and terrestrial mammals, +should not inhabit oceanic islands; and why, on the other hand, new and +peculiar species of bats, which can traverse the ocean, should so often +be found on islands far distant from any continent. Such facts +as the presence of peculiar species of bats, and the absence of all +other mammals, on oceanic islands, are utterly inexplicable on the +theory of independent acts of creation. + +The existence of closely allied or representative species in any two +areas, implies, on the theory of descent with modification, that the +same parents formerly inhabited both areas; and we almost invariably +find that wherever many closely allied species inhabit two areas, some +identical species common to both still exist. Wherever many closely +allied yet distinct species occur, many doubtful forms and varieties of +the same species likewise occur. It is a rule of high generality that +the inhabitants of each area are related to the inhabitants of the +nearest source whence immigrants might have been derived. We see this +in nearly all the plants and animals of the Galapagos archipelago, of +Juan Fernandez, and of the other American islands being related in the +most striking manner to the plants and animals of the neighbouring +American mainland; and those of the Cape de Verde archipelago and other +African islands to the African mainland. It must be admitted that these +facts receive no explanation on the theory of creation. + +The fact, as we have seen, that all past and present organic beings +constitute one grand natural system, with group subordinate to group, +and with extinct groups often falling in between recent groups, is +intelligible on the theory of natural selection with its contingencies +of extinction and divergence of character. On these same principles we +see how it is, that the mutual affinities of the species and genera +within each class are so complex and circuitous. We see why certain +characters are far more serviceable than others for classification;—why +adaptive characters, though of paramount importance to the being, are +of hardly any +importance in classification; why characters derived from rudimentary +parts, though of no service to the being, are often of high +classificatory value; and why embryological characters are the most +valuable of all. The real affinities of all organic beings are due to +inheritance or community of descent. The natural system is a +genealogical arrangement, in which we have to discover the lines of +descent by the most permanent characters, however slight their vital +importance may be. + +The framework of bones being the same in the hand of a man, wing of a +bat, fin of the porpoise, and leg of the horse,—the same number of +vertebræ forming the neck of the giraffe and of the elephant,—and +innumerable other such facts, at once explain themselves on the theory +of descent with slow and slight successive modifications. The +similarity of pattern in the wing and leg of a bat, though used for +such different purpose,—in the jaws and legs of a crab,—in the petals, +stamens, and pistils of a flower, is likewise intelligible on the view +of the gradual modification of parts or organs, which were alike in the +early progenitor of each class. On the principle of successive +variations not always supervening at an early age, and being inherited +at a corresponding not early period of life, we can clearly see why the +embryos of mammals, birds, reptiles, and fishes should be so closely +alike, and should be so unlike the adult forms. We may cease marvelling +at the embryo of an air-breathing mammal or bird having branchial slits +and arteries running in loops, like those in a fish which has to +breathe the air dissolved in water, by the aid of well-developed +branchiæ. + +Disuse, aided sometimes by natural selection, will often tend to reduce +an organ, when it has become useless by changed habits or under changed +conditions +of life; and we can clearly understand on this view the meaning of +rudimentary organs. But disuse and selection will generally act on each +creature, when it has come to maturity and has to play its full part in +the struggle for existence, and will thus have little power of acting +on an organ during early life; hence the organ will not be much reduced +or rendered rudimentary at this early age. The calf, for instance, has +inherited teeth, which never cut through the gums of the upper jaw, +from an early progenitor having well-developed teeth; and we may +believe, that the teeth in the mature animal were reduced, during +successive generations, by disuse or by the tongue and palate having +been fitted by natural selection to browse without their aid; whereas +in the calf, the teeth have been left untouched by selection or disuse, +and on the principle of inheritance at corresponding ages have been +inherited from a remote period to the present day. On the view of each +organic being and each separate organ having been specially created, +how utterly inexplicable it is that parts, like the teeth in the +embryonic calf or like the shrivelled wings under the soldered +wing-covers of some beetles, should thus so frequently bear the plain +stamp of inutility! Nature may be said to have taken pains to reveal, +by rudimentary organs and by homologous structures, her scheme of +modification, which it seems that we wilfully will not understand. + +I have now recapitulated the chief facts and considerations which have +thoroughly convinced me that species have changed, and are still slowly +changing by the preservation and accumulation of successive slight +favourable variations. Why, it may be asked, have all the most eminent +living naturalists and geologists rejected this view of the mutability +of species? It cannot be +asserted that organic beings in a state of nature are subject to no +variation; it cannot be proved that the amount of variation in the +course of long ages is a limited quantity; no clear distinction has +been, or can be, drawn between species and well-marked varieties. It +cannot be maintained that species when intercrossed are invariably +sterile, and varieties invariably fertile; or that sterility is a +special endowment and sign of creation. The belief that species were +immutable productions was almost unavoidable as long as the history of +the world was thought to be of short duration; and now that we have +acquired some idea of the lapse of time, we are too apt to assume, +without proof, that the geological record is so perfect that it would +have afforded us plain evidence of the mutation of species, if they had +undergone mutation. + +But the chief cause of our natural unwillingness to admit that one +species has given birth to other and distinct species, is that we are +always slow in admitting any great change of which we do not see the +intermediate steps. The difficulty is the same as that felt by so many +geologists, when Lyell first insisted that long lines of inland cliffs +had been formed, and great valleys excavated, by the slow action of the +coast-waves. The mind cannot possibly grasp the full meaning of the +term of a hundred million years; it cannot add up and perceive the full +effects of many slight variations, accumulated during an almost +infinite number of generations. + +Although I am fully convinced of the truth of the views given in this +volume under the form of an abstract, I by no means expect to convince +experienced naturalists whose minds are stocked with a multitude of +facts all viewed, during a long course of years, from a point of view +directly opposite to mine. It is so easy +to hide our ignorance under such expressions as the “plan of creation,” +“unity of design,” etc., and to think that we give an explanation when +we only restate a fact. Any one whose disposition leads him to attach +more weight to unexplained difficulties than to the explanation of a +certain number of facts will certainly reject my theory. A few +naturalists, endowed with much flexibility of mind, and who have +already begun to doubt on the immutability of species, may be +influenced by this volume; but I look with confidence to the future, to +young and rising naturalists, who will be able to view both sides of +the question with impartiality. Whoever is led to believe that species +are mutable will do good service by conscientiously expressing his +conviction; for only thus can the load of prejudice by which this +subject is overwhelmed be removed. + +Several eminent naturalists have of late published their belief that a +multitude of reputed species in each genus are not real species; but +that other species are real, that is, have been independently created. +This seems to me a strange conclusion to arrive at. They admit that a +multitude of forms, which till lately they themselves thought were +special creations, and which are still thus looked at by the majority +of naturalists, and which consequently have every external +characteristic feature of true species,—they admit that these have been +produced by variation, but they refuse to extend the same view to other +and very slightly different forms. Nevertheless they do not pretend +that they can define, or even conjecture, which are the created forms +of life, and which are those produced by secondary laws. They admit +variation as a _vera causa_ in one case, they arbitrarily reject it in +another, without assigning any distinction in the two cases. The day +will come when this will be given as a curious illustration of +the blindness of preconceived opinion. These authors seem no more +startled at a miraculous act of creation than at an ordinary birth. But +do they really believe that at innumerable periods in the earth’s +history certain elemental atoms have been commanded suddenly to flash +into living tissues? Do they believe that at each supposed act of +creation one individual or many were produced? Were all the infinitely +numerous kinds of animals and plants created as eggs or seed, or as +full grown? and in the case of mammals, were they created bearing the +false marks of nourishment from the mother’s womb? Although naturalists +very properly demand a full explanation of every difficulty from those +who believe in the mutability of species, on their own side they ignore +the whole subject of the first appearance of species in what they +consider reverent silence. + +It may be asked how far I extend the doctrine of the modification of +species. The question is difficult to answer, because the more distinct +the forms are which we may consider, by so much the arguments fall away +in force. But some arguments of the greatest weight extend very far. +All the members of whole classes can be connected together by chains of +affinities, and all can be classified on the same principle, in groups +subordinate to groups. Fossil remains sometimes tend to fill up very +wide intervals between existing orders. Organs in a rudimentary +condition plainly show that an early progenitor had the organ in a +fully developed state; and this in some instances necessarily implies +an enormous amount of modification in the descendants. Throughout whole +classes various structures are formed on the same pattern, and at an +embryonic age the species closely resemble each other. Therefore I +cannot doubt that the theory of descent with modification +embraces all the members of the same class. I believe that animals have +descended from at most only four or five progenitors, and plants from +an equal or lesser number. + +Analogy would lead me one step further, namely, to the belief that all +animals and plants have descended from some one prototype. But analogy +may be a deceitful guide. Nevertheless all living things have much in +common, in their chemical composition, their germinal vesicles, their +cellular structure, and their laws of growth and reproduction. We see +this even in so trifling a circumstance as that the same poison often +similarly affects plants and animals; or that the poison secreted by +the gall-fly produces monstrous growths on the wild rose or oak-tree. +Therefore I should infer from analogy that probably all the organic +beings which have ever lived on this earth have descended from some one +primordial form, into which life was first breathed. + +When the views entertained in this volume on the origin of species, or +when analogous views are generally admitted, we can dimly foresee that +there will be a considerable revolution in natural history. +Systematists will be able to pursue their labours as at present; but +they will not be incessantly haunted by the shadowy doubt whether this +or that form be in essence a species. This I feel sure, and I speak +after experience, will be no slight relief. The endless disputes +whether or not some fifty species of British brambles are true species +will cease. Systematists will have only to decide (not that this will +be easy) whether any form be sufficiently constant and distinct from +other forms, to be capable of definition; and if definable, whether the +differences be sufficiently important to deserve a specific name. This +latter point will become a far more essential consideration +than it is at present; for differences, however slight, between any two +forms, if not blended by intermediate gradations, are looked at by most +naturalists as sufficient to raise both forms to the rank of species. +Hereafter we shall be compelled to acknowledge that the only +distinction between species and well-marked varieties is, that the +latter are known, or believed, to be connected at the present day by +intermediate gradations, whereas species were formerly thus connected. +Hence, without quite rejecting the consideration of the present +existence of intermediate gradations between any two forms, we shall be +led to weigh more carefully and to value higher the actual amount of +difference between them. It is quite possible that forms now generally +acknowledged to be merely varieties may hereafter be thought worthy of +specific names, as with the primrose and cowslip; and in this case +scientific and common language will come into accordance. In short, we +shall have to treat species in the same manner as those naturalists +treat genera, who admit that genera are merely artificial combinations +made for convenience. This may not be a cheering prospect; but we shall +at least be freed from the vain search for the undiscovered and +undiscoverable essence of the term species. + +The other and more general departments of natural history will rise +greatly in interest. The terms used by naturalists of affinity, +relationship, community of type, paternity, morphology, adaptive +characters, rudimentary and aborted organs, etc., will cease to be +metaphorical, and will have a plain signification. When we no longer +look at an organic being as a savage looks at a ship, as at something +wholly beyond his comprehension; when we regard every production of +nature as one which has had a history; when we contemplate every +complex structure +and instinct as the summing up of many contrivances, each useful to the +possessor, nearly in the same way as when we look at any great +mechanical invention as the summing up of the labour, the experience, +the reason, and even the blunders of numerous workmen; when we thus +view each organic being, how far more interesting, I speak from +experience, will the study of natural history become! + +A grand and almost untrodden field of inquiry will be opened, on the +causes and laws of variation, on correlation of growth, on the effects +of use and disuse, on the direct action of external conditions, and so +forth. The study of domestic productions will rise immensely in value. +A new variety raised by man will be a far more important and +interesting subject for study than one more species added to the +infinitude of already recorded species. Our classifications will come +to be, as far as they can be so made, genealogies; and will then truly +give what may be called the plan of creation. The rules for classifying +will no doubt become simpler when we have a definite object in view. We +possess no pedigrees or armorial bearings; and we have to discover and +trace the many diverging lines of descent in our natural genealogies, +by characters of any kind which have long been inherited. Rudimentary +organs will speak infallibly with respect to the nature of long-lost +structures. Species and groups of species, which are called aberrant, +and which may fancifully be called living fossils, will aid us in +forming a picture of the ancient forms of life. Embryology will reveal +to us the structure, in some degree obscured, of the prototypes of each +great class. + +When we can feel assured that all the individuals of the same species, +and all the closely allied species of most genera, have within a not +very remote period descended +from one parent, and have migrated from some one birthplace; and when +we better know the many means of migration, then, by the light which +geology now throws, and will continue to throw, on former changes of +climate and of the level of the land, we shall surely be enabled to +trace in an admirable manner the former migrations of the inhabitants +of the whole world. Even at present, by comparing the differences of +the inhabitants of the sea on the opposite sides of a continent, and +the nature of the various inhabitants of that continent in relation to +their apparent means of immigration, some light can be thrown on +ancient geography. + +The noble science of Geology loses glory from the extreme imperfection +of the record. The crust of the earth with its embedded remains must +not be looked at as a well-filled museum, but as a poor collection made +at hazard and at rare intervals. The accumulation of each great +fossiliferous formation will be recognised as having depended on an +unusual concurrence of circumstances, and the blank intervals between +the successive stages as having been of vast duration. But we shall be +able to gauge with some security the duration of these intervals by a +comparison of the preceding and succeeding organic forms. We must be +cautious in attempting to correlate as strictly contemporaneous two +formations, which include few identical species, by the general +succession of their forms of life. As species are produced and +exterminated by slowly acting and still existing causes, and not by +miraculous acts of creation and by catastrophes; and as the most +important of all causes of organic change is one which is almost +independent of altered and perhaps suddenly altered physical +conditions, namely, the mutual relation of organism to organism,—the +improvement of one being entailing the improvement or the extermination +of +others; it follows, that the amount of organic change in the fossils of +consecutive formations probably serves as a fair measure of the lapse +of actual time. A number of species, however, keeping in a body might +remain for a long period unchanged, whilst within this same period, +several of these species, by migrating into new countries and coming +into competition with foreign associates, might become modified; so +that we must not overrate the accuracy of organic change as a measure +of time. During early periods of the earth’s history, when the forms of +life were probably fewer and simpler, the rate of change was probably +slower; and at the first dawn of life, when very few forms of the +simplest structure existed, the rate of change may have been slow in an +extreme degree. The whole history of the world, as at present known, +although of a length quite incomprehensible by us, will hereafter be +recognised as a mere fragment of time, compared with the ages which +have elapsed since the first creature, the progenitor of innumerable +extinct and living descendants, was created. + +In the distant future I see open fields for far more important +researches. Psychology will be based on a new foundation, that of the +necessary acquirement of each mental power and capacity by gradation. +Light will be thrown on the origin of man and his history. + +Authors of the highest eminence seem to be fully satisfied with the +view that each species has been independently created. To my mind it +accords better with what we know of the laws impressed on matter by the +Creator, that the production and extinction of the past and present +inhabitants of the world should have been due to secondary causes, like +those determining the birth and death of the individual. When I view +all beings not as special creations, but as the lineal descendants of +some few beings which lived long before the +first bed of the Silurian system was deposited, they seem to me to +become ennobled. Judging from the past, we may safely infer that not +one living species will transmit its unaltered likeness to a distant +futurity. And of the species now living very few will transmit progeny +of any kind to a far distant futurity; for the manner in which all +organic beings are grouped, shows that the greater number of species of +each genus, and all the species of many genera, have left no +descendants, but have become utterly extinct. We can so far take a +prophetic glance into futurity as to foretel that it will be the common +and widely-spread species, belonging to the larger and dominant groups, +which will ultimately prevail and procreate new and dominant species. +As all the living forms of life are the lineal descendants of those +which lived long before the Silurian epoch, we may feel certain that +the ordinary succession by generation has never once been broken, and +that no cataclysm has desolated the whole world. Hence we may look with +some confidence to a secure future of equally inappreciable length. And +as natural selection works solely by and for the good of each being, +all corporeal and mental endowments will tend to progress towards +perfection. + +It is interesting to contemplate an entangled bank, clothed with many +plants of many kinds, with birds singing on the bushes, with various +insects flitting about, and with worms crawling through the damp earth, +and to reflect that these elaborately constructed forms, so different +from each other, and dependent on each other in so complex a manner, +have all been produced by laws acting around us. These laws, taken in +the largest sense, being Growth with Reproduction; Inheritance which is +almost implied by reproduction; Variability from the indirect and +direct action of the external conditions +of life, and from use and disuse; a Ratio of Increase so high as to +lead to a Struggle for Life, and as a consequence to Natural Selection, +entailing Divergence of Character and the Extinction of less-improved +forms. Thus, from the war of nature, from famine and death, the most +exalted object which we are capable of conceiving, namely, the +production of the higher animals, directly follows. There is grandeur +in this view of life, with its several powers, having been originally +breathed into a few forms or into one; and that, whilst this planet has +gone cycling on according to the fixed law of gravity, from so simple a +beginning endless forms most beautiful and most wonderful have been, +and are being, evolved. + + + + +INDEX. + + +Aberrant groups, 429. + +Abyssinia, plants of, 375. + +Acclimatisation, 139. + +Affinities: +of extinct species, 329. +of organic beings, 411. + +Agassiz: +on Amblyopsis, 139. +on groups of species suddenly appearing, 302, 305. +on embryological succession, 338. +on the glacial period, 366. +on embryological characters, 418. +on the embryos of vertebrata, 439. +on parallelism of embryological development and geological succession, +449. + +Algæ of New Zealand, 376. + +Alligators, males, fighting, 88. + +Amblyopsis, blind fish, 139. + +America, North: +productions allied to those of Europe, 371. +boulders and glaciers of, 373. +South, no modern formations on west coast, 290. + +Ammonites, sudden extinction of, 321. + +Anagallis, sterility of, 247. + +Analogy of variations, 159. + +Ancylus, 386. + +Animals: +not domesticated from being variable, 17. +domestic, descended from several stocks, 19. +acclimatisation of, 141. +of Australia, 116. +with thicker fur in cold climates, 133. +blind, in caves, 137. +extinct, of Australia, 339. + +Anomma, 240. + +Antarctic islands, ancient flora of, 399. + +Antirrhinum, 161. + +Ants: +attending aphides, 211. +slave-making instinct, 219. + +Ants, neuter, structure of, 236. + +Aphides attended by ants, 211. + +Aphis, development of, 442. + +Apteryx, 182. + +Arab horses, 35. + +Aralo-Caspian Sea, 339. + +Archiac, M. de, on the succession of species, 325. + +Artichoke, Jerusalem, 142. + +Ascension, plants of, 389. + +Asclepias, pollen of, 193. + +Asparagus, 359. + +Aspicarpa, 417. + +Asses, striped, 163. + +Ateuchus, 135. + +Audubon: +on habits of frigate-bird, 185. +on variation in birds’-nests, 212. +on heron eating seeds, 387. + +Australia: +animals of, 116. +dogs of, 215. +extinct animals of, 339. +European plants in, 375. + +Azara on flies destroying cattle, 72. + +Azores, flora of, 363. + +Babington, Mr., on British plants, 48. + +Balancement of growth, 147. + +Bamboo with hooks, 197. + +Barberry, flowers of, 98. + +Barrande, M.: +on Silurian colonies, 313. +on the succession of species, 325. +on parallelism of palæozoic formations, 328. +on affinities of ancient species, 330. + +Barriers, importance of, 347. + +Batrachians on islands, 393. + +Bats: +how structure acquired, 180. +distribution of, 394. + +Bear, catching water-insects, 184. + +Bee: +sting of, 202. +queen, killing rivals, 202. + +Bees fertilising flowers, 73. + +Bees: +hive, not sucking the red clover, 95. +cell-making instinct, 224. +humble, cells of, 225. +parasitic, 218. + +Beetles: +wingless, in Madeira, 135. +with deficient tarsi, 135. + +Bentham, Mr.: +on British plants, 48. +on classification, 419. + +Berkeley, Mr., on seeds in salt-water, 358. + +Bermuda, birds of, 391. + +Birds: +acquiring fear, 212. +annually cross the Atlantic, 364. +colour of, on continents, 132. +fossil, in caves of Brazil, 339. +of Madeira, Bermuda, and Galapagos, 390. +song of males, 89. +transporting seeds, 361. +waders, 386. +wingless, 134, 182. +with traces of embryonic teeth, 451. + +Bizcacha, 349. +affinities of, 429. + +Bladder for swimming in fish, 190. + +Blindness of cave animals, 137, + +Blyth, Mr.: +on distinctness of Indian cattle, 18. +on striped Hemionus, 163. +on crossed geese, 253. + +Boar, shoulder-pad of, 88. + +Borrow, Mr., on the Spanish pointer, 35. + +Bory St. Vincent on Batrachians, 393. + +Bosquet, M., on fossil Chthamalus, 304. + +Boulders, erratic, on the Azores, 363. + +Branchiæ, 190. + +Brent, Mr.: +on house-tumblers, 214. +on hawks killing pigeons, 362. + +Brewer, Dr., on American cuckoo, 217. + +Britain, mammals of, 395. + +Bronn on duration of specific forms, 293. + +Brown, Robert, on classification, 414. + +Buckman on variation in plants, 10. + +Buzareingues on sterility of varieties, 270. + +Cabbage, varieties of, crossed, 99. + +Calceolaria, 251. + +Canary-birds, sterility of hybrids, 252. + +Cape de Verde islands, 398. + +Cape of Good Hope, plants of, 110, 375. + +Carrier-pigeons killed by hawks, 362. + +Cassini on flowers of compositæ, 145. + +Catasetum, 424. + +Cats: +with blue eyes, deaf, 12. +variation in habits of, 91. +curling tail when going to spring, 201. + +Cattle: +destroying fir-trees, 71. +destroyed by flies in La Plata, 72. +breeds of, locally extinct, 111. +fertility of Indian and European breeds, 254. + +Cave, inhabitants of, blind, 137. + +Centres of creation, 352. + +Cephalopodæ, development of, 442. + +Cervulus, 253. + +Cetacea, teeth and hair, 144. + +Ceylon, plants of, 375. + +Chalk formation, 322. + +Characters: +divergence of, 111. +sexual, variable, 156. +adaptive or analogical, 427. + +Charlock, 76. + +Checks: +to increase, 67. +mutual, 71. + +Chickens, instinctive tameness of, 216. + +Chthamalinæ, 288. + +Chthamalus, cretacean species of, 304. + +Circumstances favourable: +to selection of domestic products, 40. +to natural selection, 101. + +Cirripedes: +capable of crossing, 101. +carapace aborted, 148. +their ovigerous frena, 192. +fossil, 304. +larvæ of, 440. + +Classification, 413. + +Clift, Mr., on the succession of types, 339. + +Climate: +effects of, in checking increase of beings, 68. +adaptation of, to organisms, 139. + +Cobites, intestine of, 190. + +Cockroach, 76. + +Collections, palæontological, poor, 287. + +Colour: +influenced by climate, 132. +in relation to attacks by flies, 198. + +Columba livia, parent of domestic pigeons, 23. + +Colymbetes, 386. + +Compensation of growth, 147. + +Compositæ: +outer and inner florets of, 144. +male flowers of, 451. + +Conclusion, general, 480. + +Conditions, slight changes in, favourable to fertility, 267. + +Coot, 185. + +Coral: +islands, seeds drifted to, 360. +reefs, indicating movements of earth, 309. + +Corn-crake, 185. + +Correlation: +of growth in domestic productions, 11. +of growth, 143, 198. + +Cowslip, 49. + +Creation, single centres of, 352. + +Crinum, 250. + +Crosses, reciprocal, 258. + +Crossing: +of domestic animals, importance in altering breeds, 20. +advantages of, 96. +unfavourable to selection, 102. + +Crustacea of New Zealand, 376. + +Crustacean, blind, 137. + +Cryptocerus, 238. + +Ctenomys, blind, 137. + +Cuckoo, instinct of, 216. + +Currants, grafts of, 262. + +Currents of sea, rate of, 359. + +Cuvier: +on conditions of existence, 206. +on fossil monkeys, 303. + +Cuvier, Fred., on instinct, 208. + +Dana, Professor: +on blind cave-animals, 139. +on relations of crustaceans of Japan, 372. +on crustaceans of New Zealand, 376. + +De Candolle: +on struggle for existence, 62. +on umbelliferæ, 146. +on general affinities, 430. + +De Candolle, Alph.: +on low plants, widely dispersed, 406. +on widely-ranging plants being variable, 53. +on naturalisation, 115. +on winged seeds, 146. +on Alpine species suddenly becoming rare, 175. +on distribution of plants with large seeds, 360. +on vegetation of Australia, 379. +on fresh-water plants, 386. +on insular plants, 389. + +Degradation of coast-rocks, 282. + +Denudation: +rate of, 285. +of oldest rocks, 308. + +Development of ancient forms, 336. + +Devonian system, 334. + +Dianthus, fertility of crosses, 256. + +Dirt on feet of birds, 362. + +Dispersal: +means of, 356. +during glacial period, 365. + +Distribution: +geographical, 346. +means of, 356. + +Disuse, effects of, under nature, 134. + +Divergence of character, 111. + +Division, physiological, of labour, 115. + +Dogs: +hairless, with imperfect teeth, 12. +descended from several wild stocks, 18. +domestic instincts of, 213. +inherited civilisation of, 215. +fertility of breeds together, 254. +of crosses, 268. +proportions of, when young, 444. + +Domestication, variation under, 7. + +Downing, Mr., on fruit-trees in America, 85. + +Downs, North and South, 285. + +Dragon-flies, intestines of, 190. + +Drift-timber, 360. + +Driver-ant, 240. + +Drones killed by other bees, 202. + +Duck: +domestic, wings of, reduced, 11. +logger-headed, 182. + +Duckweed, 385. + +Dugong, affinities of, 414. + +Dung-beetles with deficient tarsi, 135. + +Dyticus, 386. + +Earl, Mr. W., on the Malay Archipelago, 395. + +Ears: +drooping, in domestic animals, 11. +rudimentary, 454. + +Earth, seeds in roots of trees, 361. + +Eciton, 238. + +Economy of organisation, 147. + +Edentata: +teeth and hair, 144. +fossil species of, 339. + +Edwards, Milne: +on physiological divisions of labour, 115. +on gradations of structure, 194. +on embryological characters, 418. + +Eggs, young birds escaping from, 87. + +Electric organs, 192. + +Elephant: +rate of increase, 64. +of glacial period, 141. + +Embryology, 439. + +Existence: +struggle for, 60. +conditions of, 206. + +Extinction: +as bearing on natural selection, 109. +of domestic varieties, 111. +317. + +Eye: +structure of, 187. +correction for aberration, 202. + +Eyes reduced in moles, 137. + +Fabre, M., on parasitic sphex, 218. + +Falconer, Dr.: +on naturalization of plants in India, 65. +on fossil crocodile, 313. +on elephants and mastodons, 334. +and Cautley on mammals of sub-Himalayan beds, 340. + +Falkland Island, wolf of, 393. + +Faults, 285. + +Faunas, marine, 348. + +Fear, instinctive, in birds, 212. + +Feet of birds, young molluscs adhering to, 385. + +Fertility: +of hybrids, 249. +from slight changes in conditions, 267. +of crossed varieties, 267. + +Fir-trees: +destroyed by cattle, 71. +pollen of, 203. + +Fish: +flying, 182. +teleostean, sudden appearance of, 305. +eating seeds, 362, 387. +fresh-water, distribution of, 384. + +Fishes: +ganoid, now confined to fresh water, 107. +electric organs of, 192. +ganoid, living in fresh water, 321. +of southern hemisphere, 376. + +Flight, powers of, how acquired, 182. + +Flowers: +structure of, in relation to crossing, 97. +of compositæ and umbelliferæ, 144. + +Forbes, E.: +on colours of shells, 132. +on abrupt range of shells in depth, 175. +on poorness of palæontological collections, 287. +on continuous succession of genera, 316. +on continental extensions, 357. +on distribution during glacial period, 366 +on parallelism in time and space, 409. + +Forests, changes in, in America, 74. + +Formation, Devonian, 334. + +Formations: +thickness of, in Britain, 284. +intermittent, 290. + +Formica rufescens, 219. + +Formica sanguinea, 219. + +Formica flava, neuter of, 239. + +Frena, ovigerous, of cirripedes, 192. + +Fresh-water productions, dispersal of, 383. + +Fries on species in large genera being closely allied to other species, +57. + +Frigate-bird, 185. + +Frogs on islands, 393. + +Fruit-trees: +gradual improvement of, 37. +in United States, 85. +varieties of, acclimatised in United States, 142. + +Fuci, crossed, 258. + +Fur, thicker in cold climates, 133. + +Furze, 439. + +Galapagos Archipelago: +birds of, 390. +productions of, 398, 400. + +Galeopithecus, 181. + +Game, increase of, checked by vermin, 68. + +Gärtner: +on sterility of hybrids, 247, 255. +on reciprocal crosses, 258. +on crossed maize and verbascum, 270. +on comparison of hybrids and mongrels, 272. + +Geese: +fertility when crossed, 253. +upland, 185. + +Genealogy important in classification, 425. + +Geoffrey St. Hilaire: +on balancement, 147. +on homologous organs, 434. + +Geoffrey St. Hilaire, Isidore: +on variability of repeated parts, 149. +on correlation in monstrosities, 11. +on correlation, 144. +on variable parts being often monstrous, 155. + +Geographical distribution, 346. + +Geography, ancient, 487. + +Geology: +future progress of, 487. +imperfection of the record, 279. + +Giraffe, tail of, 195. + +Glacial period, 365. + +Gmelin on distribution, 365. + +Gnathodon, fossil, 368. + +Godwin-Austen, Mr., on the Malay Archipelago, 299. + +Goethe on compensation of growth, 147. + +Gooseberry, grafts of, 262. + +Gould, Dr. A., on land-shells, 397. + +Gould, Mr.: +on colours of birds, 132. +on birds of the Galapagos, 398. +on distribution of genera of birds, 404. + +Gourds, crossed, 270. + +Grafts, capacity of, 261. + +Grasses, varieties of, 113. + +Gray, Dr. Asa: +on trees of United States, 100. +on naturalised plants in the United States, 115. +on rarity of intermediate varieties, 176. +on Alpine plants, 365. + +Gray, Dr. J. E., on striped mule, 165. + +Grebe, 185. + +Groups, aberrant, 429. + +Grouse: +colours of, 84. +red, a doubtful species, 49. + +Growth: +compensation of, 147. +correlation of, in domestic products, 11. +correlation of, 143. + +Habit: +effect of, under domestication, 11. +effect of, under nature, 134. +diversified, of same species, 183. + +Hair and teeth, correlated, 144. + +Harcourt, Mr. E. V., on the birds of Madeira, 391. + +Hartung, M., on boulders in the Azores, 363. + +Hazel-nuts, 359. + +Hearne on habits of bears, 184. + +Heath, changes in vegetation, 72, + +Heer, O., on plants of Madeira, 107. + +Helix pomatia, 397. + +Helosciadium, 359. + +Hemionus, striped, 163. + +Herbert, W.: +on struggle for existence, 62. +on sterility of hybrids, 249. + +Hermaphrodites crossing, 96. + +Heron eating seed, 387. + +Heron, Sir R., on peacocks, 89. + +Heusinger on white animals not poisoned by certain plants, 12. + +Hewitt, Mr., on sterility of first crosses, 264. + +Himalaya: +glaciers of, 373. +plants of, 375. + +Hippeastrum, 250. + +Holly-trees, sexes of, 93. + +Hollyhock, varieties of, crossed, 271. + +Hooker, Dr., on trees of New Zealand, 100. + +Hooker, Dr.: +on acclimatisation of Himalayan trees, 140. +on flowers of umbelliferæ, 145. +on glaciers of Himalaya, 373. +on algæ of New Zealand, 376. +on vegetation at the base of the Himalaya, 378. +on plants of Tierra del Fuego, 374, 378. +on Australian plants, 375, 399. +on relations of flora of South America, 379. +on flora of the Antarctic lands, 381, 399. +on the plants of the Galapagos, 391, 398. + +Hooks: +on bamboos, 197. +to seeds on islands, 392. + +Horner, Mr., on the antiquity of Egyptians, 18. + +Horns, rudimentary, 454. + +Horse, fossil, in La Plata, 318. + +Horses: +destroyed by flies in La Plata, 72. +striped, 163. +proportions of, when young, 445. + +Horticulturists, selection applied by, 32. + +Huber on cells of bees, 230. + +Huber, P.: +on reason blended with instinct, 208. +on habitual nature of instincts, 208. +on slave making ants, 219. +on Melipona domestica, 225. + +Humble-bees, cells of, 225. + +Hunter, J., on secondary sexual characters, 150. + +Hutton, Captain, on crossed geese, 253. + +Huxley, Professor: +on structure of hermaphrodites, 101. +on embryological succession, 338. +on homologous organs, 438. +on the development of aphis, 442. + +Hybrids and mongrels compared, 272. + +Hybridism, 245. + +Hydra, structure of, 190. + +Ibla, 148. + +Icebergs transporting seeds, 363. + +Increase, rate of, 63. + +Individuals: +numbers favourable to selection, 102. +many, whether simultaneously created, 356. + +Inheritance: +laws of, 12. +at corresponding ages, 14, 86. + +Insects: +colour of, fitted for habitations, 84. +sea-side, colours of, 132. +blind, in caves, 138. +luminous, 193. +neuter, 236. + +Instinct, 207. + +Instincts, domestic, 213. + +Intercrossing, advantages of, 96. + +Islands, oceanic, 388. + +Isolation favourable to selection, 104. + +Japan, productions of, 372. + +Java, plants of, 375. + +Jones, Mr. J. M., on the birds of Bermuda, 391. + +Jussieu on classification, 417. + +Kentucky, caves of, 137. + +Kerguelen-land, flora of, 381, 399. + +Kidney-bean, acclimatisation of, 142. + +Kidneys of birds, 144. + +Kirby on tarsi deficient in beetles, 135. + +Knight, Andrew, on cause of variation, 7. + +Kölreuter: +on the barberry, 98. +on sterility of hybrids, 247. +on reciprocal crosses, 258. +on crossed varieties of nicotiana, 271. +on crossing male and hermaphrodite flowers, 451. + +Lamarck on adaptive characters, 427. + +Land-shells: +distribution of, 397. +of Madeira, naturalised, 402. + +Languages, classification of, 422. + +Lapse, great, of time, 282. + +Larvæ, 440. + +Laurel, nectar secreted by the leaves, 92. + +Laws of variation, 131. + +Leech, varieties of, 76. + +Leguminosæ, nectar secreted by glands, 92. + +Lepidosiren, 107, 330. + +Life, struggle for, 60. + +Lingula, Silurian, 306. + +Linnæus, aphorism of, 413. + +Lion: +mane of, 88. +young of, striped, 439. + +Lobelia fulgens, 73, 98. + +Lobelia, sterility of crosses, 250. + +Loess of the Rhine, 384. + +Lowness of structure connected with variability, 149. + +Lowness, related to wide distribution, 406. + +Lubbock, Mr., on the nerves of coccus, 46. + +Lucas, Dr. P.: +on inheritance, 12. +on resemblance of child to parent, 275. + +Lund and Clausen on fossils of Brazil, 339. + +Lyell, Sir C.: +on the struggle for existence, 62. +on modern changes of the earth, 95. +on measure of denudation, 283. +on a carboniferous land-shell, 289. +on fossil whales, 303. +on strata beneath Silurian system, 307. +on the imperfection of the geological record, 310. +on the appearance of species, 312. +on Barrande’s colonies, 313. +on tertiary formations of Europe and North America, 323. +on parallelism of tertiary formations, 328. +on transport of seeds by icebergs, 363. +on great alternations of climate, 382. +on the distribution of fresh-water shells, 385. +on land-shells of Madeira, 402. + +Lyell and Dawson on fossilized trees in Nova Scotia, 296. + +Macleay on analogical characters, 427. + +Madeira: +plants of, 107. +beetles of, wingless, 135. +fossil land-shells of, 339. +birds of, 390. + +Magpie tame in Norway, 212. + +Maize, crossed, 270. + +Malay Archipelago: +compared with Europe, 299. +mammals of, 395. + +Malpighiaceæ, 417. + +Mammæ, rudimentary, 451. + +Mammals: +fossil, in secondary formation, 303. +insular, 393. + +Man, origin of races of, 199. + +Manatee, rudimentary nails of, 454. + +Marsupials: +of Australia, 116. +fossil species of, 339. + +Martens, M., experiment on seeds, 360. + +Martin, Mr. W. C., on striped mules, 165. + +Matteuchi on the electric organs of rays, 193. + +Matthiola, reciprocal crosses of, 258. + +Means of dispersal, 356. + +Melipona domestica, 225. + +Metamorphism of oldest rocks, 308. + +Mice: +destroying bees, 74. +acclimatisation of, 141. + +Migration, bears on first appearance of fossils, 296. + +Miller, Professor, on the cells of bees, 226. + +Mirabilis, crosses of, 258. + +Missel-thrush, 76. + +Misseltoe, complex relations of, 3. + +Mississippi, rate of deposition at mouth, 284. + +Mocking-thrush of the Galapagos, 402. + +Modification of species, how far applicable, 483. + +Moles, blind, 137. + +Mongrels: +fertility and sterility of, 267. +and hybrids compared, 272. + +Monkeys, fossil, 303, + +Monocanthus, 424. + +Mons, Van, on the origin of fruit-trees, 29, 39. + +Moquin-Tandon on sea-side plants, 132. + +Morphology, 434. + +Mozart, musical powers of, 209. + +Mud, seeds in, 386. + +Mules, striped, 165. + +Müller, Dr. F., on Alpine Australian plants, 375. + +Murchison, Sir R.: +on the formations of Russia, 289. +on azoic formations, 307. +on extinction, 317. + +Mustela vison, 179. + +Myanthus, 424. + +Myrmecocystus, 238. + +Myrmica, eyes of, 240. + +Nails, rudimentary, 453. + +Natural history: +future progress of, 484. +selection, 80. +system, 413. + +Naturalisation: +of forms distinct from the indigenous species, 115. +in New Zealand, 201. + +Nautilus, Silurian, 306. + +Nectar of plants, 92. + +Nectaries, how formed, 92. + +Nelumbium luteum, 387. + +Nests, variation in, 212. + +Neuter insects, 236. + +Newman, Mr., on humble-bees, 74. + +New Zealand: +productions of, not perfect, 201. +naturalised products of, 337. +fossil birds of, 339. +glacial action in, 373. +crustaceans of, 376. +algæ of, 376. +number of plants of, 389. +flora of, 399. + +Nicotiana: +crossed varieties of, 271. +certain species very sterile, 257. + +Noble, Mr., on fertility of Rhododendron, 251. + +Nodules, phosphatic, in azoic rocks, 307. + +Oak, varieties of, 50. + +Onites apelles, 135. + +Orchis, pollen of, 193. + +Organs: +of extreme perfection, 186. +electric, of fishes, 192. +of little importance, 194. +homologous, 434. +rudiments of, 450. + +Ornithorhynchus, 107, 416. + +Ostrich: +not capable of flight, 134. +habit of laying eggs together, 218. +American, two species of, 349. + +Otter, habits of, how acquired, 179. + +Ouzel, water, 185. + +Owen, Professor: +on birds not flying, 134. +on vegetative repetition, 149. +on variable length of arms in ourang-outang, 150. +on the swim-bladder of fishes, 191. +on electric organs, 192. +on fossil horse of La Plata, 319. +on relations of ruminants and pachyderms, 329. +on fossil birds of New Zealand, 339. +on succession of types, 339. +on affinities of the dugong, 414. +on homologous organs, 435. +on the metamorphosis of cephalopods and spiders, 442. + +Pacific Ocean, faunas of, 348. + +Paley on no organ formed to give pain, 201. + +Pallas on the fertility of the wild stocks of domestic animals, 253. + +Paraguay, cattle destroyed by flies, 72. + +Parasites, 217. + +Partridge, dirt on feet, 362. + +Parts: +greatly developed, variable, 150. +degrees of utility of, 201. + +Parus major, 183. + +Passiflora, 251. + +Peaches in United States, 85. + +Pear, grafts of, 261. + +Pelargonium: +flowers of, 145. +sterility of, 251. + +Pelvis of women, 144. + +Peloria, 145. + +Period, glacial, 365. + +Petrels, habits of, 184. + +Phasianus, fertility of hybrids, 253. + +Pheasant, young, wild, 216. + +Philippi on tertiary species in Sicily, 312. + +Pictet, Professor: +on groups of species suddenly appearing, 302, 305. +on rate of organic change, 313. +on continuous succession of genera, 316. +on close alliance of fossils in consecutive formations, 335. +on embryological succession, 338. + +Pierce, Mr., on varieties of wolves, 91. + +Pigeons: +with feathered feet and skin between toes, 12. +breeds described, and origin of, 20. +breeds of, how produced, 39, 42. +tumbler, not being able to get out of egg, 87. +reverting to blue colour, 160. +instinct of tumbling, 214. +carriers, killed by hawks, 362. +young of, 445. + +Pistil, rudimentary, 451. + +Plants: +poisonous, not affecting certain coloured animals, 12. +selection applied to, 32. +gradual improvement of, 37. +not improved in barbarous countries, 38. +destroyed by insects, 67. +in midst of range, have to struggle with other plants, 77. +nectar of, 92. +fleshy, on sea-shores, 132. +fresh-water, distribution of, 386. +low in scale, widely distributed, 406. + +Plumage, laws of change in sexes of birds, 89. + +Plums in the United States, 85. + +Pointer dog: +origin of, 35. +habits of, 213. + +Poison not affecting certain coloured animals, 12. + +Poison, similar effect of, on animals and plants, 484. + +Pollen of fir-trees, 203, + +Poole, Col., on striped hemionus, 163. + +Potamogeton, 387. + +Prestwich, Mr., on English and French eocene formations, 328. + +Primrose, 49. +sterility of, 247. + +Primula, varieties of, 49. + +Proteolepas, 148. + +Proteus, 139. + +Psychology, future progress of, 488. + +Quagga, striped, 165. + +Quince, grafts of, 261. + +Rabbit, disposition of young, 215. + +Races, domestic, characters of, 16. + +Race-horses: +Arab, 35. +English, 356. + +Ramond on plants of Pyrenees, 368. + +Ramsay, Professor: +on thickness of the British formations, 284. +on faults, 285. + +Ratio of increase, 63. + +Rats: +supplanting each other, 76. +acclimatisation of, 141. +blind in cave, 137. + +Rattle-snake, 201. + +Reason and instinct, 208. + +Recapitulation, general, 459. + +Reciprocity of crosses, 258. + +Record, geological, imperfect, 279. + +Rengger on flies destroying cattle, 72. + +Reproduction, rate of, 63. + +Resemblance to parents in mongrels and hybrids, 273. + +Reversion: +law of inheritance, 14. +in pigeons to blue colour, 160. + +Rhododendron, sterility of, 251. + +Richard, Professor, on Aspicarpa, 417. + +Richardson, Sir J.: +on structure of squirrels, 180. +on fishes of the southern hemisphere, 376. + +Robinia, grafts of, 262. + +Rodents, blind, 137. + +Rudimentary organs, 450. + +Rudiments important for classification, 416. + +Sageret on grafts, 262. + +Salmons, males fighting, and hooked jaws of, 88. + +Salt-water, how far injurious to seeds, 358. + +Saurophagus sulphuratus, 183. + +Schiödte on blind insects, 138. + +Schlegel on snakes, 144. + +Sea-water, how far injurious to seeds, 358. + +Sebright, Sir J.: +on crossed animals, 20. +on selection of pigeons, 31. + +Sedgwick, Professor, on groups of species suddenly appearing, 302. + +Seedlings destroyed by insects, 67. + +Seeds: +nutriment in, 77. +winged, 146. +power of resisting salt-water, 358. +in crops and intestines of birds, 361. +eaten by fish, 362, 387. +in mud, 386. +hooked, on islands, 392. + +Selection: +of domestic products, 29. +principle not of recent origin, 33. +unconscious, 34. +natural, 80. +sexual, 87. +natural, circumstances favourable to, 101. + +Sexes, relations of, 87. + +Sexual: +characters variable, 156. +selection, 87. + +Sheep: +Merino, their selection, 31. +two sub-breeds unintentionally produced, 36. +mountain, varieties of, 76. + +Shells: +colours of, 132. +littoral, seldom embedded, 288. +fresh-water, dispersal of, 385. +of Madeira, 391. +land, distribution of, 397. + +Silene, fertility of crosses, 257. + +Silliman, Professor, on blind rat, 137. + +Skulls of young mammals, 197, 437. + +Slave-making instinct, 219. + +Smith, Col. Hamilton, on striped horses, 164. + +Smith, Mr. Fred.: +on slave-making ants, 219. +on neuter ants, 239. + +Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283. + +Snap-dragon, 161. + +Somerville, Lord, on selection of sheep, 31. + +Sorbus, grafts of, 262. + +Spaniel, King Charles’s breed, 35. + +Species: +polymorphic, 46. +common, variable, 53. +in large genera variable, 54. +groups of, suddenly appearing, 302, 306. +beneath Silurian formations, 306. +successively appearing, 312. +changing simultaneously throughout the world, 322. + +Spencer, Lord, on increase in size of cattle, 35. + +Sphex, parasitic, 218. + +Spiders, development of, 442. + +Spitz-dog crossed with fox, 268. + +Sports in plants, 9. + +Sprengel, C. C.: +on crossing, 98. +on ray-florets, 145. + +Squirrels, gradations in structure, 180. + +Staffordshire, heath, changes in, 72. + +Stag-beetles, fighting, 88. + +Sterility: +from changed conditions of life, 9. +of hybrids, 246. +laws of, 254. +causes of, 263. +from unfavourable conditions, 265. +of certain varieties, 269. + +St. Helena, productions of, 389. + +St. Hilaire, Aug., on classification, 418. + +St. John, Mr., on habits of cats, 91. + +Sting of bee, 202. + +Stocks, aboriginal, of domestic animals, 18, + +Strata, thickness of, in Britain, 284. + +Stripes on horses, 163. + +Structure, degrees of utility of, 201. + +Struggle for existence, 60. + +Succession, geological, 312. + +Succession of types in same areas, 338. + +Swallow, one species supplanting another, 76. + +Swim-bladder, 190. + +System, natural, 413. + +Tail: +of giraffe, 195. +of aquatic animals, 196. +rudimentary, 454. + +Tarsi deficient, 135. + +Tausch on umbelliferous flowers, 146. + +Teeth and hair: +correlated, 144. +embryonic, traces of, in birds, 451. +rudimentary, in embryonic calf, 450, 480. + +Tegetmeier, Mr., on cells of bees, 228, 233. + +Temminck on distribution aiding classification, 419. + +Thouin on grafts, 262. + +Thrush: +aquatic species of, 185. +mocking, of the Galapagos, 402. +young of, spotted, 439. +nest of, 243. + +Thuret, >M., on crossed fuci, 258. + +Thwaites, Mr., on acclimatisation, 140. + +Tierra del Fuego: +dogs of, 215. +plants of, 374, 378. + +Timber-drift, 360. + +Time, lapse of, 282. + +Titmouse, 183. + +Toads on islands, 393. + +Tobacco, crossed varieties of, 271. + +Tomes, Mr., on the distribution of bats, 394. + +Transitions in varieties rare, 172. + +Trees: +on islands belong to peculiar orders, 392. +with separated sexes, 99. + +Trifolium pratense, 73, 94. + +Trifolium incarnatum, 94. + +Trigonia, 321. + +Trilobites, 306. +sudden extinction of, 321. + +Troglodytes, 243. + +Tucutucu, blind, 137. + +Tumbler pigeons: +habits of, hereditary, 214. +young of, 446. + +Turkey-cock, brush of hair on breast, 90. + +Turkey: +naked skin on head, 197. +young, wild, 216. + +Turnip and cabbage, analogous variations of, 159. + +Type, unity of, 206. + +Types, succession of, in same areas, 338. + +Udders: +enlarged by use, 11. +rudimentary, 451. + +Ulex, young leaves of, 439. + +Umbelliferæ, outer and inner florets of, 144. + +Unity of type, 206. + +Use: +effects of, under domestication, 11. +effects of, in a state of nature, 134. + +Utility, how far important in the construction of each part, 199. + +Valenciennes on fresh-water fish, 384. + +Variability of mongrels and hybrids, 274. + +Variation: +under domestication, 7. +caused by reproductive system being affected by conditions of life, 8. +under nature, 44. +laws of, 131. + +Variations: +appear at corresponding ages, 14, 86. +analogous in distinct species, 159. + +Varieties: +natural, 44. +struggle between, 75. +domestic, extinction of, 111. +transitional, rarity of, 172. +when crossed, fertile, 267. +when crossed, sterile, 269. +classification of, 423. + +Verbascum: +sterility of, 251. +varieties of, crossed, 270. + +Verneuil, M. de, on the succession of species, 325. + +Viola tricolor, 73. + +Volcanic islands, denudation of, 284. + +Vulture, naked skin on head, 197. + +Wading-birds, 386. + +Wallace, Mr.: +on origin of species, 2. +on law of geographical distribution, 355. +on the Malay Archipelago, 395. + +Wasp, sting of, 202. + +Water, fresh, productions of, 383. + +Water-hen, 185. + +Waterhouse, Mr.: +on Australian marsupials, 116. +on greatly developed parts being variable, 150. +on the cells of bees, 225. +on general affinities, 429. + +Water-ouzel, 185. + +Watson, Mr. H. C.: +on range of varieties of British plants, 58. +on acclimatisation, 140. +on flora of Azores, 363. +on Alpine plants, 367, 376. +on rarity of intermediate varieties, 176. + +Weald, denudation of, 285. + +Web of feet in water-birds, 185. + +West Indian islands, mammals of, 395. + +Westwood: +on species in large genera being closely allied to others, 57. +on the tarsi of Engidæ, 157. +on the antennæ of hymenopterous insects, 416. + +Whales, fossil, 303. + +Wheat, varieties of, 113. + +White Mountains, flora of, 365. + +Wings, reduction of size, 134. + +Wings: +of insects homologous with branchiæ, 191. +rudimentary, in insects, 451. + +Wolf: +crossed with dog, 214. +of Falkland Isles, 393. + +Wollaston, Mr.: +on varieties of insects, 48. +on fossil varieties of land-shells in Madeira, 52. +on colours of insects on sea-shore, 132. +on wingless beetles, 135. +on rarity of intermediate varieties, 176. +on insular insects, 389. +on land-shells of Madeira, naturalised, 402. + +Wolves, varieties of, 90. + +Woodpecker: +habits of, 184. +green colour of, 197. + +Woodward, Mr.: +on the duration of specific forms, 293. +on the continuous succession of genera, 316. +on the succession of types, 339. + +World, species changing simultaneously throughout, 322. + +Wrens, nest of, 243. + +Youatt, Mr.: +on selection, 31. +on sub-breeds of sheep, 36. +on rudimentary horns in young cattle, 454. + +Zebra, stripes on, 163. + +*** END OF THE PROJECT GUTENBERG EBOOK 1228 *** |