CHAPTER 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. 
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