Physiologically, the effects of use or disuse are, in the main, effects on the relative nutrition, and hence on the differential growth of organs. When an organ is well exercised, there is increased nutrition and increased growth of tissue, muscular, nervous, glandular, or other. When an organ is, so to speak, neglected, there is diminished blood-supply, diminished growth, and diminished functional power. The development of a complex activity would necessitate a complex adjustment of size and efficiency of parts, involving a nice balance of differential growth dependent on delicately regulated nutrition. What is the evidence that adjusted nutrition can be inherited?

With regard to man, there is some evidence which bears upon this subject. Mr. Arbuthnot Lane, in his valuable papers in the Journal of Anatomy and Physiology, has shown that certain occupations, such as shoemaking, coal-heaving, etc., produce recognizable effects upon the skeleton, the muscular system, and other parts of the organization. And he believes[CD] that such effects are inherited, being very much more marked in the third generation than they were in the first. Sir William Turner informed Professor Herdman that, in his opinion, the peculiar habits of a tribe, such as tree-climbing among the Australians, or those natives of the interior of New Guinea whose houses are built in the upper branches of lofty trees, not only affect each generation individually, but have an intensified action through the influence of heredity.[CE]

Mr. Francis Galton's results mainly deal with human faculty; and though faculty has undoubtedly an organic basis, I do not propose to consider the evidence afforded by instinct, intelligence, or intellectual faculties in this chapter. Mention should, however, be made of the interesting results of his study of twins. Twins are either of the same sex, in which case they are remarkably alike, or of different sexes, in which case they are apt to differ even more widely than is usual with brothers and sisters. The former are believed to be developed from one ovum which has divided into two halves, each of which has given rise to a distinct individual; the latter from two different ova. Mr. Galton collected a large mass of statistics concerning twins of both classes. The result of this analysis seems to be that, in the case of "identical twins," the resemblances are not superficial, but extremely intimate; that they are not apt to be modified to any large extent by the circumstances of life; that where marked diversity sets in it is due to some form of illness; and, on the whole, that innate tendencies outmaster acquired modifications. "Nature is far stronger than nurture within the limited range that I have been careful to assign to the latter." On the other hand, speaking of dissimilar twins, Mr. Galton says, "I have not a single case in which my correspondents speak of originally dissimilar characters having become assimilated through identity of nurture." "The impression that all this evidence leaves on the mind is one of some wonder whether nurture can do anything at all, beyond giving instruction and professional training." "There is no escape from the conclusion that nature prevails enormously over nurture where the differences of nurture do not exceed what is commonly to be found among persons of the same rank of society and in the same country."[CF]

Combining the results of Messrs. Lane and Galton, we may say that it requires persistent and long-continued influence to modify the individual, and change, even by a little, the structure inherited or given by nature; but that if this structure is thus modified, there may be a tendency for such modification to increase by hereditary summation of effects. We require, however, further and fuller observations to render the evidence of such hereditary summation to any extent convincing.

Turning now from the evidence afforded by man[CG] to that afforded by animals, we may consider first that presented by domesticated breeds. They might be expected to afford exceptionally good examples. Their modifiability and the readiness with which they interbreed are two of the determining causes of their selection for domestication. They have, moreover, been placed under new conditions of life, and they undoubtedly exhibit changes of structure, many of which Darwin[CH] regarded as attributable to the effects of use and disuse. In domestic ducks, the relative weight and strength of the wing-bones have been diminished, while conversely the weight and strength of the leg-bones have been increased. The bones of the shoulder-girdle have been decreased in weight and "the prominence of the crest of the sternum, relatively to its length, is also much reduced in all the domestic breeds. These changes," says Darwin, "have evidently been caused by the lessened use of the wings." The shoulder-girdle and breast-bone of domestic fowls have been similarly reduced. After a careful consideration of numerous facts concerning the brains of rabbits, Darwin concluded that this "most important and complicated organ in the whole organization is subject to the law of decrease in size from disuse." And Sir J. Crichton Browne has recently shown that, in the wild duck, the brain is nearly twice as heavy in proportion to the body as it is in the comparatively imbecile domestic duck. In pigs, the nature of the food supplied during many generations has apparently affected the length of the intestines; for, according to Cuvier, their length to that of the body in the wild boar is as 9 to 1, in the common domestic boar as 13.5 to 1, and in the Siam breed as 16 to 1. With regard to horses, Darwin tells us that "veterinarians are unanimous that horses are affected with spavins, splints, ring-bones, etc., from being shod and from travelling on hard roads, and they are almost unanimous that a tendency to these malformations is transmitted."

These are samples of the effects of domestication. It has been suggested, however, that, quite apart from any diminution from disuse, the reduction of size in parts or organs may be the result of the absence or cessation of selection. If an organ be subject to selection, the mean size in adult creatures will be that of the selected individuals; but if selection ceases, it will be the mean of those born. Let us suppose that nine individuals are born, and that the size of some organ varies in these from 1, the most efficient, to 9, the least efficient. The birth-mean will therefore be, as shown on the left-hand side of the following table, at the level of number 5, four being more efficient, and four less efficient. But if, of these nine, six be eliminated, then the mean of the survivals will be as shown on the right-hand side of the table:—

1
2 —Survival-mean.
3
4
Birth-mean—5
6 Eliminated individuals.
7
8
9

The result, then, of the cessation of selection will be to reduce the survival-mean to the birth-mean, and that without any necessary effect of disuse. But unless this be accompanied by a tendency to diminution due to economy of growth or some other cause, this cannot produce any well-marked or considerable amount of reduction. I very much question, for example, whether the cessation of selection, even with the co-operation of the principle of economy of growth, will adequately account for the reduction to nearly one-half its original proportion of the brain of the duck. The subject will be more fully discussed, however, in the next chapter.

There is probably but little tendency for disused parts to be reduced in size through artificial selection. An imbecile duck does not probably taste nicer than one with bigger brains. On the other hand, the increase of size in organs may presumably, in certain cases, be increased by selection. Pigs, for example, have been selected according to their fattening capacity. Those with longer intestines, and therefore increased absorbent surface, may well have an advantage in this respect. Hence, in selecting pigs for fattening, breeders may have been unconsciously selecting those with the longest intestines. Of course, on this view, the longer intestine must be there to be selected, and the increased length must be due to variation. But this may be all-round variation (cause unknown), not variation in one direction, the result of increased function.

Another point that has to be taken into consideration is the amount of individual increment or decrement, owing to individual use or disuse, apart from any possible summation of results.