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. But serpents, having acquired the habit of gliding along the ground, and concealing themselves amid the grass, their bodies, as a consequence of constantly repeated efforts to lengthen themselves out in order to pass through narrow passages, have acquired considerable length of body which is out of all proportion to their breadth. Now, feet would have been useless to these animals, and consequently would have remained unemployed; for long legs would have interfered with their desire to go on their bellies; and short legs, being limited in number to four, would have been incapable of moving their bodies. Thus total disuse among these races of animals has caused the parts which have fallen into disuse totally to disappear. Many insects, which by their order and genus should have wings, lack them more or less completely for similar reasons. _III.--The Advantages of Use_