« ПредыдущаяПродолжить »
selection" fulfils and exhausts its office, not by fostering but by checking deflexion in structure. Those savage Red Indians who are said to have cast their young boys into a river, leaving those to perish who could not save themselves by swimming, would doubtless rear thoroughbred Red Indians : no more : not a race with a promise of fins. Just so the tested buffalo will grow up rigidly herbivorous ; nor will the lion be set a-learning to eat
straw like the ox.
15. Varieties will, indeed, arise which will be tran ient or permanent, according to definite and intelligible conditions. Permanent varieties spring from permanent causes--change of climate, supply of food, or the interference of man. Each of these, or any two, or all three of them combined, will produce, and that within a very limited space of time, extreme, though still superficial, diversities in size, colour, strength, and direction of natural instincts. 15 But none of these causes singly, nor all of them together, will work change in the dental formula, or the dorsal vertebræ, or the digestive and generative apparatus, or any other of those essential characters on which the comparative anatomist and physiologist rely. They will not disable the toxicologist from discriminating the blood-corpuscles of man from those of a deer ;16 they will not endow a dog with the retractile claw of the cat; nor a horse with canine teeth. Yet these are the only class of alterative agencies known to experience and observation. Competition, under whatever name disguised --- Natural Selection, or Struggle for Life---is not one of them. Certain changes in the superficial structure and habits of animals result from, and are perpetuated by, temperature, food, or artificial interference ; but never, so far as we know, from free comity and intercourse, under like conditions, in a state of nature.
16. Transient peculiarities, it is true, will occur in individual animals--such as the short legs in Seth Wright's ram, born 1791, and selected, as disabled from overleaping fences, as progenitor of the otter-breed. In the same way, men and women have been born with six fingers. But, along with this licence of rare and casual divergence, there is a law of constant revergence by which it is effectually controlled. Save by artificial fostering, such peculiarities cannot hold their ground. Every other member of the species in which they appear is arrayed against them, and in league to obliterate them. There is no variety of six-fingered men; the sixth finger, or rather bud, for it is not a true digit
having no metacarpal socket, like the phalanges proper ---lingers on against hopeless odds through a few generations; at last it is absorbed and conquered. 17 Nature, so to say, sets her face against it; and puts it under ban as a fugitive monstrosity. The ancon breed of sheep, when it ceased to be profitable, and so lost its artificial guardianship, rapidly merged in the general
Under the same law of revergence, dogs, on lapsing into the wild state, lose the habit of barking and only howl.18 So the wild boar of South America, after many centuries of intermediate domestication, has dropped the mask it was made to wear, and reappeared in the likeness of its prototype in the forest of Ardennes 19 Through constant milking, the ordinary do
. mestic cow has come to have teats larger than in proportion, and the secretion of milk is perpetual. In Columbia, from the size of the farms, the practice of milking was laid aside. With what result ? “ In a few generations, the natural structure of parts and state of the function was restored. If the calf dies, the milk ceases to flow; and it is only by keeping him with his dam by day that an opportunity of obtaining milk from cows by night can be found.” 20 In such ways, nature shows her fundamental conservatism, and fidelity to primordial form. Permitted perturbation is kept in check by the over-ruling law of equilibrium and recovery. Man's experiments on the lower creatures, then, so far from yielding an analogy to the methods of nature, present the strongest possible contrast. It is by suspending and crossing those methods that such experiments succeed. Man is a reformer, often a deformer, and his influence over animals is innovation ; it is the sole aim of nature to keep them up to the mark. She may be repulsed or counteracted, tamen usque recurret. We talk of the “unchanging east.” Nature, save by the Creative will and interference, is more changeless than the unchanging east of man.
17. If we must apply the same name, then, to quite distinct and antagonistic processes, let it not be forgotten that natural “selection” is ever tending to efface such exceptional peculiarities in animal structure as human selection strives to cherish and perpetuate. All variation, moreover, howsoever brought about, is transacted within inflexible limits. Let the “magician's wand” be waved ever so dexterously, a sheep is still a sheep. In six years, we are assured,21 Sir John Sebright will undertake to modify to pattern the head and beak of a pigeon: in how many more will he engage to turn the
milk which it is the wonderful prerogative of this family to secrete in the “crop," during the breeding season, for after mixture with the food of its young, into mammalian channels ? For be it likewise carefully noted that such variation as is proved to be attainable, whether through human experiments, or under changed outward conditions, is not of the nature of ascensive development. To alter is not necessarily to elevate. To cultivate one point in an animal artificially, is to sacrifice another ; and what the breeder calls “improvement” the naturalist deems deformity. Adaptive diversity may depress below the old level, but it will not promote to a new.
At best it imports a power of conforming without injury to altered circumstances; but much more frequently degradation under hardships. Take the most decisive of examples, that of man himself. No conceivable change in food or climate will materially improve a robust and well-fed race. On the other hand, stinted food brings stunted stature. In Sligo and northern Mayo ;-“Five feet two inches upon an average, pot-bellied, bow-legged, abortively featured, their clothing a wisp of rags ; these spectres of a people that two centuries ago were wellgrown, able-bodied and comely, stalk abroad into the daylight of civilization, the annual apparitions of Irish