If it is supposed, he continues, that these laws are hypothetical, they may be demonstrated by the following facts: The adult baleen whale is without teeth, although in the fœtus teeth are present, concealed in the jaws. The loss of the teeth is the result of the whale swallowing its food without first masticating it. The ant-eater is also without teeth, and has also the habit of swallowing its food without chewing it. The mole has very small eyes, and this is the result of its having made very little use of them, since its habits are subterranean. Another animal, the aspalax, has only the rudiments of eyes, and has almost completely lost the power of sight. This animal also lives underground like the mole.
Proteus, an aquatic salamander living in deep caves, has only rudimentary eyes. In these latter cases it is the disuse of the eye that has led to its degeneration. This is proven, Lamarck adds, by the fact that the organs of hearing are never in this condition, because sound vibrations penetrate everywhere, even into the densest bodies.
It is a part of the plan of organization of the reptiles that they have four legs; but the snakes, although belonging to this group, have no legs. This absence of legs is explained by their having acquired the habit of gliding over the ground, and of concealing themselves in the grass. Owing to their repeated effort to elongate themselves, in order to pass through narrow spaces, their bodies have become drawn out. Under these circumstances legs would be useless, since long ones would interfere with their motion, and short ones could not move their long bodies. Since the plan of organization limits the snakes to only four legs, and since this number would be useless, they have disappeared.
Many insects are destitute of wings, although wings are a part of the plan of organization of this group. They are absent only in those forms whose habits render wings useless, consequently they have disappeared through disuse.
The preceding cases are those in which the disuse of an organ has led to its degeneration. The following cases are cited to show that by use an organ increases in size. The formation of the web in the feet of water-birds has already been given as a character which Lamarck supposes to have been acquired through use; also the case of shore-birds, which, by an effort to elongate their legs, have actually made them so in the course of time. The necks of water-birds are also long on account of their having been stretched in the efforts to catch fish. The long tongues of the ant-eater, of the woodpecker, and of humming-birds are the result of use, and the long, forked tongue of serpents has come from their using their tongue to feel objects in front of them.