t 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_

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. Similar