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Now-a-days most naturalists are more Darwinian than Mr. Darwin himself. I do not mean that their beliefs in organic evolution are more decided; though I shall be supposed to mean this by the mass of readers, who identify Mr. Darwin’s great contribution to the theory of organic evolution, with the theory of organic evolution itself, and even with the theory of evolution at large. But I mean that the particular factor which he first recognized as having played so immense a part in organic evolution, has come to be regarded by his followers as the sole factor, though it was not so regarded by him. It is true that he apparently rejected altogether the causal agencies alleged by earlier inquirers. In the Historical Sketch prefixed to the later editions of his Origin of Species (p. xiv, note), he writes:–“It is curious how largely my grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous grounds of opinion of Lamarck in his ‘Zoonomia’ (vol. i, pp. 500-510), published in 1794.” And since, among the views thus referred to, was the view that changes of structure in organisms arise by the inheritance of functionally-produced changes, Mr. Darwin seems, by the above sentence, to have implied his disbelief in such inheritance. But he did not mean to imply this; for his belief in it as a cause of evolution, if not an important cause, is proved by many passages in his works. In the first chapter of the Origin of Species (p. 8 of the sixth edition), he says respecting the inherited effects of habit, that “with animals the increased use or disuse of parts has had a more marked influence;” and he gives as instances the changed relative weights of the wing bones and leg bones of the wild duck and the domestic duck, “the great and inherited development of the udders in cows and goats,” and the drooping ears of various domestic animals. Here are other passages taken from the latest edition of the work.

“I think there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited” (p. 108). [And on the following pages he gives five further examples of such effects.] “Habit in producing constitutional peculiarities and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects” (p. 131). “When discussing special cases, Mr. Mivart passes over the effects of the increased use and disuse of parts, which I have always maintained to be highly important, and have treated in my ‘Variation under Domestication’ at greater length than, as I believe, any other writer” (p. 176). “Disuse, on the other hand, will account for the less developed condition of the whole inferior half of the body, including the lateral fins” (p. 188). “I may give another instance of a structure which apparently owes its origin exclusively to use or habit” (p. 188). “It appears probable that disuse has been the main agent in rendering organs rudimentary” (pp. 400-401). “On the whole, we may conclude that habit, or use and disuse, have, in some cases, played a considerable part in the modification of the constitution and structure; but that the effects have often been largely combined with, and sometimes overmastered by, the natural selection of innate variations” (p. 114).

In his subsequent work, The Variation of Animals and Plants under Domestication, where he goes into full detail, Mr. Darwin gives more numerous illustrations of the inherited effects of use and disuse. The following are some of the cases, quoted from volume i of the first edition.

Treating of domesticated rabbits, he says:–“the want of exercise has apparently modified the proportional length of the limbs in comparison with the body” (p. 116). “We thus see that the most important and complicated organ [the brain] in the whole organization is subject to the law of decrease in size from disuse” (p. 129). He remarks that in birds of the oceanic islands “not persecuted by any enemies, the reduction of their wings has probably been caused by gradual disuse.” After comparing one of these, the water-hen of Tristan d’Acunha, with the European water-hen, and showing that all the bones concerned in flight are smaller, he adds–“Hence in the skeleton of this natural species nearly the same changes have occurred, only carried a little further, as with our domestic ducks, and in this latter case I presume no one will dispute that they have resulted from the lessened use of the wings and the increased use of the legs” (pp. 286-7). “As with other long-domesticated animals, the instincts of the silk-moth have suffered. The caterpillars, when placed on a mulberry-tree, often commit the strange mistake of devouring the base of the leaf on which they are feeding, and consequently fall down; but they are capable, according to M. Robinet, of again crawling up the trunk. Even this capacity sometimes fails, for M. Martins placed some caterpillars on a tree, and those which fell were not able to remount and perished of hunger; they were even incapable of passing from leaf to leaf” (p. 304).