On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life

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“ 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.” “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.”

FELLOW OF THE ROYAL, GEOLOGICAL, LINNAIAN, ETC., SOCIETIES 5 AUTHOR OF ‘ JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE’S VOYAGE 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 te Man's power of Selection, 2 2. ls Ol. pewn ee ie4ad

Variability — Individual differences — Doubtful species — Wide ranging, much diffused, and common species vary most—Spe- cies of the larger genera in any country vary more than the species of the smaller genera—Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges .. .. 44-59 Bears on natural selection—The term used in a wide sense—Geo- metrical powers of increase — Rapid increase of naturalised animals and plants—Nature of the checks to increase—Compe- tition 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 be- tween species of the same genus—The relation of organism to organism the most important of all relations .. Page 60-79

Natural Selection —its power compared with man’s selection — its power on characters of trifling importance — its power at all ages and on both sexes — Sexual Selection — On the generality of inter- crosses between individuals of the same species — Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals — Slow action — Extinction caused by Natural Selection-— Divergence of Cha- racter, 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.) oc0 ween tie La WE Ree) ete tee

Effects of external conditions — Use and disuse, combined with natural selection; organs of flight and of vision — Acclimatisa- tion — 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 .. .. .. .. .. 181-170

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 me Fe ee ae herenree een Te OG

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 Distinction between the sterility of first crosses and of hybrids — Sterility various in degree, not universal, affected by close inter- breeding, 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 con- ditions 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 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 deposi- tion and of denudation — On the poorness of our paleeontological 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 Page 279-311 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 appear- ance 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 .. .. 812-345

Present distribution cannot be accounted for by differences in phy- sical conditions — Importance of barriers Affinity of the pro- ductions of the same continent — Centres of creation — Means of dispersal, by changes of climate and of the level of the land, aud by occasional means — Dispersal during the Glacial period co-extensive with the world .. .. .. .. .. «. 346-382 Distribution of fresh-water productions —On the inhabitants of oceanic islands — Absence of Batrachians and of terrestrial Mam- mals — 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 pre- BERT AGHAPLCES: Pes: 2-° “cases. ss ce. ‘eer, Paseyeso-410

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 se- parates and defines groups — MorrHoLoey, between members of the same class, between parts of the same individual — Empryo.oey, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age — RUDIMENTARY ORGANS ; their origin explained — Summary

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 .. .. .. « ~~. 409-490 The Diagram to front page 117, and to face the latter part Volume. pf WHEN on board H.MLS. ‘ Beagle,’ as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological rela- tions of the present to the past inhabitants of that con- tinent. 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 Lin- nean 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 me- moir, some brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I cannot here give references and autho- rities 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 here- after 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 con- ceivable that a naturalist, reflectnmg on the mutual affinities of organic beings, on their embryological rela- tions, their geographical distribution, geological succes- sion, 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 admi- ration. Naturalists continually refer to external con- ditions, such as climate, food, &c., as the only possible cause of variation. In one very limited sense, as we shall hereafter see, this may be true; but it is pre- posterous 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, un- touched and. unexplained.

It is, therefore, of the highest importance to gain a clear insight into the means of modification and co- adaptation. At the commencement of my observations it seemed to me probable that a careful study of domes- ticated 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 imvariably 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 inevi- tably 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, conse- quently, 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 corre- lation of growth. In the four succeeding chapters, the most apparent and gravest difficulties on the theory will be given: namely, first, the difficulties of transi- tions, 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 em- bryonic condition. In the last chapter I shall give a

brief recapitulation of the whole work, and a few con- cluding 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 de- liberate study and dispassionate judgment of which I am capable, that the view which most naturalists enter- tain, and which I formerly entertained—namely, that each species has been independently created—is erro- neous. Iam fully convinced that species are not im- mutable ; 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 con- vinced that Natural Selection has been the main but not exclusive means of modification.

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 con- ditions 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 ex- posed 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 improve- ment 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 con- ception. Several reasons make me believe in this; but the chief one is the remarkable effect which confine- ment or cultivation has on the functions of the repro- ductive 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. No- thing 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 con- finement 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 un- perceived causes as to fail in acting, we need not be surprised at this system, when it does act under con- finement, acting not quite regularly, and producing off- spring 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), showimg that their repro- ductive system has not been thus affected ; so will some animals and plants withstand domestication or cultiva- tion, and vary very Es hardly more than in a state of nature.