6 MAMMALS 



axis, often clothed with long hair, which may considerably exceed the total length 

 of the body. The Mammal, in which the relative length of the tail is greatest, is 

 a small one from Madagascar, belonging to the Insectivorous order, and named 

 Microgale, in which the tail is nearly three times as long as the body. In some of 

 the apes and monkeys the tail is absent; and it is very short in the bears among the 

 Carnivores, and in many deer among the Hoofed Mammals, or Ungulates. In 

 many Ungulates, however, such as cattle, it is of great length ; and in that group 

 it has its extremity furnished with a tuft of hair, and thus forms an effectual in- 

 strument for brushing away flies from the body. In the spider-monkeys of South 

 America, as well as in the opossums and phalangers, in certain porcupines, and 

 other forms, the tail is prehensile, and thus serves as an important aid in climbing, or 

 to suspend its owner's head downwards. In the beaver the tail is expanded into a 

 flattened oar-like form, which probably acts as a rudder in swimming. But the 

 most remarkable modification of this useful organ occurs in the whales and dol- 

 phins, when it is expanded into a large forked structure, termed by whalers 

 ' flukes, ' and is the main organ in propelling the body through the water. 



In regard to the external covering, we have already said that hairs 

 K erna ^^ always present on some portion of the body during some period of 



life. In the whales these hairs may, however, be reduced to a few 

 bristles in the region of the mouth, which disappear when the animal attains ma- 

 turity. Mammals never develop that modified kind of hair-structure known as 

 feathers, which are peculiar to Birds. The body may, however, be covered with 

 ^overlapping scales, like those so common in Reptiles, but this occurs only in the 

 pangolins, or scaly ant-eaters of India and Africa. The tail of the common rat is 

 an example of a part of the body covered with scales, having their edges in appo- 

 sition; but in both these instances hairs are mingled with the scales. Still rarer 

 than scales are bony plates, developed in the true skin. At the present day these 

 structures are only met with among the well-known armadillos of South America, 

 which are furnished with bucklers and transverse bands of these bony plates', and 

 are in some cases able to roll themselves up into a ball, presenting on all sides an 

 impenetrable coat of mail. In the Pleistocene, or latest geological period before the 

 present, South America produced, however, a number of huge Mammals allied to 

 the armadillos, and known as glyptodonts, which were covered with a continuous 

 cuirass of bony plates, reaching in some cases more than an inch in thickness. 

 That these huge and well-armored forms, which one might regard as typical 

 examples of animals fitted to withstand all enemies, have perished, while their 

 smaller and less completely defended allies have lived on, shows us that there are 

 other causes at work than the attacks of foes in the destruction of animals. 

 Between the plates of the armor of the armadillos hairs are always developed, and 

 in one species these are so abundant as to completely hide the plates themselves, and 

 render the general appearance that of an ordinary hairy Mammal. 



The use of hair is mainly to protect the body from cold, and thus to aid in the 

 maintenance of a uniform high temperature ; and when hairs are absent, we find this 

 function performed by a more or less thick fatty layer beneath the skin, which, 

 when it is excessively developed, as in the whales, is known as blubber. To com- 



