It might, therefore, seem hopeless to show the necessity for their existence on Darwinian principles, and to prove that they are physiologically active organs.

Nevertheless, recent investigations on this point have furnished evidence that this is possible.
"It is known that many reptiles, and above all the snakes, cast off the whole skin at once, whereas human beings do so by degrees.

If by any accident they are prevented doing so, they infallibly die, because the old skin has grown so tough and hard that it hinders the increase in volume which is inseparable from the growth of the animal.

The casting of the skin is induced by the formation on the surface of the inner epidermis, of a layer of very fine and equally distributed hairs, which evidently serve the purpose of mechanically raising the old skin by their rigidity and position.

These hairs then may be designated as _casting hairs_.