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.
The frequent use of an organ, if constant and habitual, increases its powers, develops it, and makes it acquire dimensions and potency such as are not found among animals which use it less.
Of this principle, the web-feet of some birds, the long legs and neck of the stork, are examples. Similarly, the elongated tongue of the ant-eater, and those of lizards and serpents.
Such wants, and the sustained efforts to satisfy them, have also resulted in the displacement of organs. Fishes which swim habitually in great masses of water, since they need to see right and left of them, have the eyes one upon either side of the head. Their bodies, more or less flat, according to species, have their edges perpendicular to the plane of the water; and their eyes are so placed as to be one on either side of the flattened body. But those whose habits bring them constantly to the banks, especially sloping banks, have been obliged to lie over upon the flattened surface in order to approach more nearly. In this position, in which more light falls on the upper than on the under surface, and their attention is more particularly fixed upon what is going on above than on what is going on below them, this want has forced one of the eyes to undergo a kind of displacement, and to keep the strange position which it occupies in the head of a sole or a turbot. The situation is not symmetrical because the mutation is not complete. In the case of the skate, however, it is complete; for in these fish the transverse flattening of the body is quite horizontal, no less than that of the head. And so the eyes of a skate are not only placed both of them on the upper surface, but have become symmetrical.