Naturalists have observed that the members of animals are adapted to their use, and thence have concluded hitherto that the formation of the members has led to their appropriate employment. Now, this is an error. For observation plainly shows that, on the contrary, the development of the members has been caused by their need and use; that these have caused them to come into existence where they were wanting.
But let us examine the facts which bear upon the effects of employment or disuse of organs resulting from the habits which a race has been compelled to form.
II.—The Penalties of Disuse
Permanent disuse of an organ as a consequence of acquired habits gradually impoverishes it, and in the end causes it to disappear, or even annihilates it altogether.
Thus vertebrates, which, in spite of innumerable particular distinctions, are alike in the plan of their organisation, are generally armed with teeth. Yet those of them which by circumstances have acquired the habit of swallowing their prey without mastication have been liable to leave their teeth undeveloped. Consequently, the teeth have either remained hidden between the bony plates of the jaws, or have even been, in the course of time, annihilated.
The whale was supposed to have no teeth at all till M. Geoffrey found them hidden in the jaws of the foetus. He has also found in birds the groove in which teeth might be placed, but without any trace of the teeth themselves. A similar case to that of the whale is the ant-eater (nyomecophaga), which has long given up the practice of mastication.
Eyes in the head are an essential part of the organisation of vertebrates. Yet the mole, which habitually makes no use of the sense of sight, has eyes so small that they can hardly be seen; and the aspalax, whose habits-resemble a mole's, has totally lost its sight, and shows but vestiges of eyes. So also the proteus, which inhabits dark caves under water.
In such cases, since the animals in question belong to a type of which eyes are an essential part, it is clear that the impoverishment, and even the total disappearance, of these organs are the results of long continued disuse.
With hearing, the case is otherwise. Sound traverses everything. Therefore, wherever an animal dwells it may exercise this faculty. And so no vertebrate lacks it, and we never find it re-appearing in any of the lower ranges. Sight disappears, re-appears, and disappears again, according as circumstances deny or permit its exercise.
Four legs attached to its skeleton are part of the reptile type; and serpents, particularly as between them and the fishes come the batrachians—frogs, etc.—ought to have four legs.