The Origin of Species Read online

On the poorness of our palaeontological 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 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.

  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.

  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.

  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.

  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.

  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, &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 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 treatme
nt, 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