[Many Cases of Supposed Transmission to be explained by Selection.]
It is upon these two lines that Galton and Weismann worked, and we may now follow in rough outline the evidence they adduced. Darwin had been able to explain, to universal satisfaction, the evolution of many types and varieties, as a result of selection alone; though certain cases of the supposed inheritance of use and disuse, and of acquired instincts, caused at times doubts to arise in his mind. But Weismann has questioned this inheritance, and has shown that—as Darwin himself sometimes believed—these may readily be explained by selection alone. The gradual increase, generation after generation, in the size of a useful limb or the perfection of a valuable organ of sense may readily be explained by selection. The fact that the limb or organ is of use to the race in its struggle will determine the survival of those born with these serviceable parts well-formed, and these in their turn will produce others as favourably or more favourably constituted, from whom further selection can take place. We cannot shut our eyes to the operation of selection in such instances, nor have we any reason for saying that part of the effect must be due to some other cause. In the case of organs which become useless and finally disappear in the course of generations, a selective agency will sufficiently account for this disappearance. As Darwin himself pointed out, a useless organ is an expense and a drain upon an animal’s capital; it requires blood, and its exercise uses up some of the sum total of energy the animal possesses. The truth of this can be shown experimentally, as when compensatory growth occurs in the rest of the body after amputation of a limb, or when one lung or kidney enlarges subsequently to the disease or removal of the other. In cases where an organ is useless, those who have it badly developed, and in consequence have other and useful parts more fully formed, will have a distinct advantage over those born with a well-formed but useless organ. We may thus explain the small size of the wings of the tame as compared with the wild duck, an instance in which Darwin saw difficulty in excluding inherited disuse. In this way we may explain the occurrence of the still smaller wings of the running ostrich and apteryx; also the blind fish of the Kentucky Cave, and the visionless eyes of the burrowing mole. As an illustration, an animal or man may, in this respect, be compared to an individual with a given amount of capital, who, if he spends his money in one direction, will thereby have less for another purpose. In this way a big leg may be obtained at the expense of a small arm, or a good ear be the cause of an indifferent eye.
When we turn to the question of the supposed transmission of acquired instincts and habits, we find that it is possible by means of the principle of selection, to explain some, at least, of the cases which presented difficulties to Darwin’s mind. Thus the tameness of rabbits, cats, and dogs, which animals have for countless generations been subjected to domestication, need not necessarily be accounted for by supposing that the results of training are transmitted. For it is easy to understand how those that would have rebelled most against man’s authority, and who were by nature the least tractable, would have been less cared for by man, and probably would finally have suffered extermination, while the docile received his attention, and were allowed to reach maturity and perpetuate the race. That this selection must be going on at the present time is very obvious, and as instances we may note the savage dogs that are constantly being destroyed, and the house dogs and domestic pets that are, in most cases, continually being selected from the docile animals and those of good temper. A dog that does not possess these good qualities can have no existence in town or village, and so by continual extermination of the unfriendly, the “friend of man” has gradually been evolved.
[Paucity of Experimental Evidence of such Transmission.]
Turning now to those cases in which characters can be acquired or experimentally stamped upon an individual, we find that no single reliable instance can be adduced in which transmission takes place. Mutilations have been practised upon male infants by Jews and other Semitic races for thousands of years; yet, in spite of this, the operation has still to be performed, for the lost parts appear in the offspring of to-day as in the earlier periods of their race’s history. Certain breeds of dogs and sheep have for many generations been systematically docked, and yet the young are born with as long tails as those of other breeds. Chinese women have compressed their feet from times long past in their history, yet Chinese female infants are still born with large feet, and have to undergo afresh the torture of their compression. More curious still, for it affects an organ of paramount importance, the brain, there is a tribe of Indians who flatten their heads in early life, entirely changing the shape not only of the skull, but of the brain itself, yet their children are born with normal rounded heads; the induced change is not transmitted.
It must be admitted that this evidence is pretty strong, and we need not wonder that it has produced such widespread conviction, although it has been so lately taken up by the thinking public.
[The Reproductive and the Body Cells.]
The body of a plant or animal is composed of small living bodies, most of them of microscopic size, called cells. These lead, to a certain extent, individual lives, and have individual characters, but they are built, as it were, together, like the bricks and stones of a house, to form the body. The cells are, all of them, nourished by the blood and lymph, and some are connected together by strands of connecting matter termed nerves. All the cells of the body are descendants from a single fertilised egg, which has resulted from the fusion of a paternal and maternal sexual cell. Among the cells of the body, and situated in special organs, are the sexual cells, likewise nourished by blood, but not connected by nerves with other parts of the body.