252 THE POPULAR SCIENCE MONTHLY. 



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 car- 

 ried 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 em- 

 ploy 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 trans- 

 formation 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 Avould 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 



