CHAPTER XXII 

 THE MAMMALS 



The class Mammalia takes its name from the possession 

 of mammary glands which produce milk for the young. 

 All mammals possess these glands and they are found in no 

 other group of animals; consequently their presence 

 serves to define quite precisely this class of vertebrates. 

 Mammals may in most cases be recognized by their 

 covering of hair, just as birds may be distinguished by 

 their feathers. In a few forms, such as the elephant and 

 rhinoceros, the hair is very scarce; and in the whales it has 

 almost entirely disappeared. Porcupines and hedgehogs 

 have many of the hairs modified into large, stiff spines 

 or quills, which are efficient organs of protection. Hair 

 is a product of the outer layer of the skin, the epidermis, 

 and, like feathers, it is commonly shed more or less period- 

 ically. New hairs are grown from a papilla at the base 

 of the older hairs that fall out. The nails and hoofs of 

 mammals as well as the outer covering of the horns of 

 cattle and related forms are also epidermal structures. 



Unlike birds and reptiles, mammals usually have well- 

 developed external ears, or pinnag, which are generally 

 shaped so as to catch sound, and are freely movable, as 

 may be seen readily by watching a dog, horse, or rabbit. 

 The sense of smell is generally acute and enables many spe- 

 cies to track their prey and others to detect their enemies 

 even at a considerable distance. The least whiff of a 

 human being may send a bear or deer scampering through 

 the forest long before its pursuer appears in sight. By 

 smell mammals may recognize their own kind and dis- 



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