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.