The Evolution Theory, Vol. 1 of 2

by August Weismann

After what has been said, it is hardly necessary to add that the size of the whole body differs in different races. But the differences are very considerable, for, according to Darwin, one of the largest runt-pigeons weighed exactly five times as much as one of the smallest tumblers with short forehead, and in the illustration ([Fig. 1]) the pouter looks a giant beside the little barb to its left.

Thus we see that nearly every part of the body of the pigeon has varied under domestication in the most diverse ways, and to a high degree; and the same is true of several other domesticated animals, poultry, horses, sheep, cattle, pigs, and so on, though the matter is not altogether so clear in their case, since descent from a single wild species cannot be proved, and is in many cases improbable. But in the case of pigeons this common descent is certain, and we have now to inquire in what manner all these variations from the parent form have been brought about.

The answering of this question is rendered easier by the fact that new breeds arise even now, and that, to some extent at least, they can be caused to arise, consciously and intentionally. In England, as well as in Germany and France, there are associations for the breeding of birds, and in England especially pigeon and poultry clubs are numerous and highly developed. These by no means confine themselves to simply preserving the purity of existing breeds, they are continually striving to improve them, by increasing and accentuating their characters, or even by introducing quite new qualities, and in many cases they succeed even in this last. Prizes are offered for particular new variations, and thus a spirit of rivalry is fostered among the breeders, and each strives to produce the desired character as quickly as possible. Darwin says: 'The English judges decided that the comb of the Spanish cock, which had previously hung limply down, should stand erect, and in five years this end was achieved; they ordained that hens should have beards, and six years later fifty-seven of the groups of hens exhibited at the Crystal Palace in London were bearded.' The transformation does not always come about so quickly, however; thus, for instance, it required thirteen years before a certain breed of tumblers was furnished with a white head. But the breeders cause every visible part of the body to vary as seems good to them, and within the last fifty years they have really brought about very considerable changes in many breeds. Their method of procedure is carefully to select for breeding those birds which already possess a faint beginning of the desired character. Domesticated animals have on the whole a higher degree of variability than wild species, and the breeder takes advantage of this. Suppose it is a question of adding a crown of feathers to a smooth-headed breed, a bird is chosen which has the feathers on the back of the head a little longer than usual, and mated for breeding. Among its descendants there will probably be some which also exhibit these slightly prominent feathers, and possibly there may be one or other of them which has these feathers considerably lengthened. This one is then used for breeding, and by continually proceeding thus, and selecting for breeding, from generation to generation, only the individuals which approach most nearly to the desired end, the wished-for character is at last secured.

Thus it is not by crossing of different breeds, but by a patient accumulating of insignificant little variations through many generations, that the desired transformations are brought about. That is the magic wand by means of which the expert breeder produces his different breeds, we might almost say, as the sculptor moulds and remoulds his clay model according to his fancy. Quite according to his fancy the breeder has brought about all the fantastic forms we are familiar with among pigeons, mere variations which are of no use either to the bird itself or to man, which simply gratify man's whim without in many cases even satisfying his sense of beauty. For many of the existing breeds of pigeons, hens, and other domesticated animals, are anything but beautiful, the body being often unharmonious in structure and sometimes actually monstrous.

Among pigeons, as well as among other domesticated animals, some changes have been brought about, which are not only of no use to their possessors, but would be actually disadvantageous if they were living under natural conditions. Some of the very short-billed breeds of pigeons have the bill so short and soft that the young can no longer use it to scratch and break the egg-shell, and would perish miserably if human aid were not at hand. The Yorkshire pig has become such a colossus of fat on weak, short legs, that if it were dependent on its own resources, it could not secure its food, much less escape from a beast of prey; and among horses the heavy cart-horse and the racer are alike unfit to cope with the dangers of a wild life, or the vicissitudes of weather.

Breeding has done much to bring about variations useful to man. Thus we have breeds of cattle which excel in flesh, or in milk, or as draught animals, and sheep which excel in flesh or in wool, and to what a height the perfecting of a useful quality can be brought is shown, in regard to fineness of wool, by that finest breed of sheep, the merino, which instead of the 5,500 hairs borne by the old German sheep on a square inch, possesses 48,000.

Not infrequently it is a particular stage of a species that has been bred by man, and the other stages have remained more or less unaltered. Thus it is with one of the few domesticated insects, the silk-moth. Only the cocoon is of use to man, and according to the cocoon different breeds are distinguished, differing in fineness, colour, &c.; but no breeds can be distinguished in reference to the larvæ, or the perfect insects. Among gooseberries there are about a hundred varieties distinguished according to the form, colour, size, thickness of skin, hairiness, &c., of the fruits, but the little, inconspicuous, green blossoms, of which the breeders take no account, are alike in them all. In the pansies (Viola tricolor), on the other hand, it is only by the flowers that the varieties are distinguished, while the seeds have remained alike in all.

It may be asked how it could have occurred to any one, when pigeons, for instance, first began to be domesticated, to wish to produce fantails or pouters, since he could have no mental picture of them in advance. Darwin replies to this objection, that it was not always conscious and methodical artificial selection, such as is now practised, that brought about the origin of breeds, but that they have very often resulted, and at first perhaps always, from unconscious selection. When savages tamed a dog, they used the 'best' of their dogs for breeding, that is, they chose those which had in the highest degree the qualities they valued, watchfulness, for instance, or if the dog were intended for the chase, keen scent and swiftness. In this way the body of the animal would be changed in a definite direction, especially if rivalry helped, and if it was the ambition of each to possess a dog as good as, or better than those of his tribal companions. That perfectly definite changes in bodily form can thus be brought about unconsciously is well illustrated by the case of a racehorse. This has arisen within the last two hundred years simply because the fleetest of the products of crossing between the Arab and the English horse were always chosen for breeding. It could not have been predicted that horses with thin neck, small head, long rump, and slender legs would necessarily be the swiftest runners; but this is the form which has resulted from the selection,—a very ugly, but very swift horse. This unconscious selection must undoubtedly have played a large part in the early stages of the evolution of the breeds of our domestic animals.

But even in the fully conscious and methodical selective breeding of particular characters, the breeder rarely alters only the one his attention is fixed on; generally quite a number of other characters alter apart from his intention as an inevitable accompaniment of the desired variation on which attention was riveted. There are breeds of rabbits whose ears hang limply down instead of standing erect, and in these so-called lop-eared rabbits the ear-muscles are partly degenerated, and as a consequence of this lack of muscular strain the skull has assumed another form. Thus the variation of one part may influence the development of a second and a third organ, and may even not stop there, for very often the influence has penetrated much deeper and affected quite remote parts of the body.

If any one were to succeed in adding a heavy pair of horns to a breed of hornless sheep, there would run parallel with the course of this variation, which was directly aimed at, a long series of secondary changes which would affect at least the whole of the anterior half of the body; the skull would become thicker and stronger to support the weight of the heavy horns; the neck-tendon (ligamentum nuchæ) would have to become thicker to hold up the heavy head, and so also with the muscles of the neck; the spinous processes of the cervical and dorsal vertebrae would become longer and stronger, and the forelegs, too, would need to adapt themselves to the heavier burden. Every organism thus resembles, as it were, a mosaic, out of which no individual group of pieces can be taken and replaced by another without in some measure disturbing the correlation and harmony of the whole: in order to restore this, the pieces all round about the changed part must be moved or replaced by others.