above all with the opossum, which uses its tail quite as much as any monkey uses its hands. Accordingly, its surface is quite bare from end to end, and in some species scaly—a fact which is rendered more comprehensible when we remember that the young opossums are carried on their mother's back, and hold themselves in that position by curling their tails around hers.
A few more special facts help to bear out the same generalization. In the gorilla, according to Du Chaillu, "the skin on the back of the fingers, near the middle phalanx, is callous and very thick, which shows that the most usual mode of progression of the animal is on all fours and resting on the knuckles." The ornithorhyncus has a flat tail, on which it leans for support, and this, says Mr. Waterhouse, "is short, depressed, and very broad, and covered with coarse hairs; these, however, are generally worn off on the under side of the tail in adult or aged individuals, probably by the friction of the ground." The toes of the very large fore-feet, used in burrowing, are also naked, as are the similar organs in the mole and many other creatures of like habit. The beaver likewise uses his tail as a support, flaps it much in the water, and is said, perhaps not quite erroneously, to employ it as a trowel in constructing his dams; and this tail is entirely devoid of hair, being covered instead with a coat of scales. We can hardly avoid being struck in this instance, as in that of some seals' and sea-lions' flappers, with the analogy of the penguin's wings, which are employed like fins in diving, and have undergone a similar transformation of their feathers into a scale-like form. In the ground kangaroos, which use the tail as a support trailing behind them on the ground, that organ is again only slightly covered with coarse hairs, almost entirely wanting on the extremity of the under surface; but in the tree-kangaroos, which carry the tail partly erect, it assumes a bushly and ornamental appearance. Like differences occur between the rats and mice on the one hand and the squirrels on the other. In those monkeys which, like Macacus brunneus, sit upon their tails, that organ is also bare. To multiply further instances would only prove tedious.
Again, when we look at the only mammals besides man which have denuded themselves of their hairy covering, we find that a great majority of them are water-frequenters. The most completely aquatic mammals, like the whales, porpoises, dugongs, and manatees, though differing widely in structure, are alike in the almost total absence of hair, while the hippopotamus is likewise a smooth-skinned animal. Now, the friction of water is of course far stronger than that of air, and it would seem to have resulted in the total depilation of these very aquatic species. Other less confirmed water-haunters, such as seals and otters, have very close fur, which scarcely at all retards them in their movements when swimming. The elephant and rhinoceros are, indeed, difficult cases to explain; but of course it is not necessary
