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The Origin of Species, Page 3

Charles Darwin

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, &c., 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 Muller 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, &c.,

  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 deciding 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, &c.; 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, &c.,

  appearing in several members of the same family. If strange and rare

  deviations of structure are truly inherited, less strange and commoner

p; 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 a
nd 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, &c., 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