The World's Greatest Books — Volume 15 — Science

by Unknown

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. 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.

Serpents need principally to see things above them, and, in response to this need, the eyes are placed so high up at the sides of the head that they can see easily what is above them on either side, while they can see in front of them but a very little distance. To compensate for this, the tongue, with which they test bodies in their line of march, has been rendered by this habit thin, long, and very contractile, and even, in most species, has been split so as to be able to test more than one object at a time. The same custom has resulted similarly in the formation of an opening at the end of the muzzle by which the tongue may be protruded without any necessity for the opening of the jaws.

The effect of use is curiously illustrated in the form and figure of the giraffe. This animal, the largest of mammals, is found in the interior of Africa, where the ground is scorched and destitute of grass, and has to browse on the foliage of trees. From the continual stretching thus necessitated over a great space of time in all the individuals of the race, it has resulted that the fore legs have become longer than the hind legs, and that the neck has become so elongated that the giraffe, without standing on its hind legs, can raise its head to a height of nearly twenty feet. Observation of all animals will furnish similar examples.

None, perhaps, is more striking than that of the kangaroo. This animal, which carries its young in an abdominal pouch, has acquired the habit of carrying itself upright upon its hind legs and tail, and of moving from place to place in a series of leaps, during which, in order not to hurt its little ones, it preserves its upright posture. Observe the result.