Charles Darwin

“ With the exception of fertility, domestic varieties resemble species when crossed in transmitting their characters in the same unequal manner to their offspring, in being subject to the prepotency of one form over the other, and in their liability to reversion. By repeated crosses a variety or a species may be made completely to absorb another. Varieties, as we shall see when we treat of their antiquity, sometimes inherit their new characters almost, or even quite, as firmly as species. With both, the conditions leading to variability and the laws governing its nature appear to be the same. ”

“ Man can create not only anomalous breeds, but others with their whole structure admirably co-ordinated for certain purposes, such as the race-horse and dray-horse, or the greyhound. It is by no means necessary that each small change of structure throughout the body, leading towards excellence, should simultaneously arise and be selected. Although man seldom attends to differences in organs which are important under a physiological point of view, yet he has so profoundly modified some breeds, that assuredly, if found wild, they would be ranked under distinct genera. ”

“ The best proof of what selection has effected is perhaps afforded by the fact that whatever part or quality in any animal, and more especially in any plant, is most valued by man, that part or quality differs most in the several races. This result is well seen by comparing the amount of difference between the fruits produced by the varieties of the same fruit-tree, between the flowers of the varieties in our flower-garden, between the seeds, roots, or leaves of our culinary and agricultural plants, in comparison with the other and not valued parts of the same plants. ”

“ If we assume that each particular variation was from the beginning of all time preordained, the plasticity of organisation, which leads to many injurious deviations of structure, as well as that redundant power of reproduction which inevitably leads to a struggle for existence, and, as a consequence, to the natural selection or survival of the fittest, must appear to us superfluous laws of nature. On the other hand, an omnipotent and omniscient Creator ordains everything and foresees everything. ”

“ And the long-continued accumulation of beneficial variations will infallibly lead to structures as diversified, as beautifully adapted for various purposes, and as excellently co-ordinated, as we see in the animals and plants all around us. Hence I have spoken of selection as the paramount power, whether applied by man to the formation of domestic breeds, or by nature to the production of species. ”

“ It matters not under what climate, or for what purpose, they are kept, whether as food for man or beast, for draught or hunting, for clothing or mere pleasure,—under all these circumstances domesticated animals and plants have varied to a much greater extent than the forms which in a state of nature are ranked as one species. Why certain animals and plants have varied more under domestication than others we do not know, any more than why some are rendered more sterile than others under changed conditions of life. ”

“ It is not surprising, seeing the great difference between many domestic breeds, that some few naturalists have concluded that all are descended from distinct aboriginal stocks, more especially as the principle of selection has been ignored, and the high antiquity of man, as a breeder of animals, has only recently become known. Most naturalists, however, freely admit that various extremely dissimilar breeds are descended from a single stock, although they do not know much about the art of breeding, cannot show the connecting links, nor say where and when the breeds arose. ”

“ Yet these same naturalists will declare, with an air of philosophical caution, that they can never admit that one natural species has given birth to another until they behold all the transitional steps. ”

“ Domestic varieties can be classed in groups under groups, like species under genera, and these under families and orders; and the classification may be either artificial,—that is, founded on any arbitrary character,—or natural. With varieties a natural classification is certainly founded, and with species is apparently founded, on community of descent, together with the amount of modification which the forms have undergone. ”

“ The fluctuating, and, as far as we can judge, never-ending variability of our domesticated productions,—the plasticity of their whole organisation,—is one of the most important facts which we learn from the numerous details given in the earlier chapters of this work. Yet domesticated animals and plants can hardly have been exposed to greater changes in their conditions than have many natural species during the incessant geological, geographical, and climatal changes of the whole world. ”

“ Although every variation is either directly or indirectly caused by some change in the surrounding conditions, we must never forget that the nature of the organisation which is acted on essentially governs the result. Distinct organisms, when placed under similar conditions, vary in different manners, whilst closely-allied organisms under dissimilar conditions often vary in nearly the same manner. We see this in the same modification frequently reappearing at long intervals of time in the same variety, and likewise in the several striking cases given of analogous or parallel varieties. ”

“ But if we give up the principle in one case,—if we do not admit that the variations of the primeval dog were intentionally guided in order that the greyhound, for instance, that perfect image of symmetry and vigour, might be formed,—no shadow of reason can be assigned for the belief that variations, alike in nature and the result of the same general laws, which have been the groundwork through natural selection of the formation of the most perfectly adapted animals in the world, man included, were intentionally and specially guided. ”

“ It is no exaggeration to assert that, if the use of the talons and tusks of a carnivorous animal, or the use of the viscid threads of a spider's web, or of the plumes and hooks on a seed may be safely inferred from their structure, we may with equal safety infer that many flowers are constructed for the express purpose of ensuring a cross with a distinct plant. ”

“ As man has domesticated so many animals and plants belonging to widely different classes, and as he certainly did not with prophetic instinct choose those species which would vary most, we may infer that all natural species, if subjected to analogous conditions, would, on an average, vary to the same degree. Few men at the present day will maintain that animals and plants were created with a tendency to vary, which long remained dormant, in order that fanciers in after ages might rear, for instance, curious breeds of the fowl, pigeon, or canary-bird. ”

“ The man who first observed and preserved a pigeon with its œsophagus a little enlarged, its beak a little longer, or its tail a little more expanded than usual, never dreamed that he had made the first step in the creation of the pouter, carrier, and fantail-pigeon. ”

“ Should the owner observe any slight variation in one of his birds, and wish to obtain a breed thus characterised, he would succeed in a surprisingly short time by carefully selecting and pairing the young. As any part which has once varied generally goes on varying in the same direction, it is easy, by continually preserving the most strongly marked individuals, to increase the amount of difference up to a high, predetermined standard of excellence. ”

“ We must not exaggerate the importance of the definite action of changed conditions in modifying all the individuals of the same species in the same manner, or of use and disuse. As every part of the organisation is highly variable, and as variations are so easily selected, both consciously and unconsciously, it is very difficult to distinguish between the effects of the selection of indefinite variations, and the direct action of the conditions of life. ”

“ Why with closely allied species their reproductive systems should almost invariably have been modified in so peculiar a manner as to be mutually incapable of acting on each other—though in unequal degrees in the two sexes, as shown by the difference in fertility between reciprocal crosses in the same species—we do not know, but may with much probability infer the cause to be as follows. Most natural species have been habituated to nearly uniform conditions of life for an incomparably longer period of time than have domestic races ”

“ This has actually occurred with strains or families of cattle, sheep, and pigeons, which have been long kept and carefully attended to by different breeders without any wish on their part to form new and distinct sub-breeds. This unconscious kind of selection will more especially come into action with animals which are highly serviceable to man; for every one tries to get the best dog, horse, cow, or sheep, and these animals will transmit more or less surely their good qualities to their offspring. Hardly any one is so careless as to breed from his worst animals. ”

“ I have heard one, who maintained that our fowls were the descendants of at least half-a-dozen aboriginal species, protest that he was in no way concerned with the origin of pigeons, ducks, rabbits, horses, or any other animal. They overlook the improbability of many species having been domesticated at an early and barbarous period. They do not consider the improbability of species having existed in a state of nature which, if like our present domestic breeds, would have been highly abnormal in comparison with all their congeners. ”

“ In accordance with the views maintained by me in this work and elsewhere, not only the various domestic races, but the most distinct genera and orders within the same great class,—for instance, whales, mice, birds, and fishes—are all the descendants of one common progenitor, and we must admit that the whole vast amount of difference between these forms of life has primarily arisen from simple variability. ”

“ We may, also, reasonably suspect, from what we see in natural species of the variability of extremely modified parts, that any structure, after remaining constant during a long series of generations, would, under new and changed conditions of life, recommence its course of variability, and might again be acted on by selection. ”

“ It is a much more important distinction that some breeds have been from their first origin modified in so slow and insensible a manner, that if we could see their early progenitors we should hardly be able to say when or how the breed first arose; whilst other breeds have originated from a strongly-marked or semi-monstrous deviation of structure, which, however, may subsequently have been augmented by selection. ”

“ But we frequently judge of the amount of variation by the production of numerous and diversified races, and we can clearly see why in many cases this has not occurred, namely, because slight successive variations have not been steadily accumulated; and such variations will never be accumulated when an animal or plant is not closely observed, or much valued, or kept in large numbers. ”

“ But fanciers have used exactly the same language with respect to domestic breeds; thus an author of an excellent treatise says he will never allow that carrier and fantail pigeons are the descendants of the wild rock-pigeon, until the transitions have "actually been observed, and can be repeated whenever man chooses to set about the task." No doubt it is difficult to realise that slight changes added up during long centuries can produce such results; but he who wishes to understand the origin of domestic breeds or natural species must overcome this difficulty. ”

“ The degree of distinctness between the various domestic breeds depends on the amount of modification which they have undergone, and especially on the neglect and final extinction of the linking, intermediate, and less valued forms.
It has often been argued that no light is thrown, from the admitted changes of domestic races, on the changes which natural species are believed to undergo, as the former are said to be mere temporary productions, always reverting, as soon as they become feral, to their pristine form. ”