CHAPTEE XVIII 



THE MAMMALS 



211. General characteristics. The mammals, constituting 

 the last and highest class of the vertebrates, comprise such 

 forms as the opossum and kangaroo, the whales and por- 

 poises, hoofed and clawed animals, the monkeys and man. 

 All are warm-blooded, air-breathing animals, having the 

 skin more or less hairy. The young are born alive, except 

 in the very lowest forms, which lay eggs like reptiles, and 

 for some time after birth are nourished by milk supplied 

 from the mammary glands (hence the word mammals) of 

 the mother. The skeleton is firm, the skull and brain 

 within are relatively large, and, with few exceptions, four 

 limbs are present. 



Most of the mammals inhabit dry land. A number, 

 however, such as the whales and seals, are aquatic ; while 

 others, such as the beavers, muskrats, etc., though not 

 especially adapted for an aquatic life, are, nevertheless, 

 active swimmers, and spend much of their time in the 

 water. 



Mammals tend to associate in companies, as we may 

 witness among the ground-squirrels, prairie-dogs, rats, 

 mice, and the seals and whales. In many cases they band 

 for mutual protection, and often fight desperately for one 

 another. Claws, hoofs, and nails are efficient weapons, and 

 spiny hairs, as on the porcupines, bony plates, such as 

 encircle the bodies of the armadillos, and thick skin and 

 hair, serve as a protection. The hair is also frequently 

 colored to harmonize the animal with its surroundings. 



225 



