CHAPTER XVI 



MAMMALS 



The mammals constitute the highest group of animals, 

 inchiding man, the monkeys and apes, the bird-like bats 

 and fish-like seals and whales, and all the various beasts we 

 commonly call quadrupeds ; altogether about 2, 500 known 

 species. They are found in all parts of the world except 

 on a few small South Sea islands. The name mammals 

 is derived from the mammary or milk glands which enable 

 the mothers to suckle their young. In size mammals 

 range from the tiny pigmy shrew and harvest-mouse 

 which can climb a stem of wheat, to the great sulphur- 

 bottom whale of the Pacific Ocean, that attains a length 

 of a hundred feet and a weight of many tons. Mammals 

 differ from fishes and batrachians and agree with reptiles 

 and birds in never having external gills ; they differ from 

 reptiles and agree with birds in being warm-blooded and 

 in having a heart with two distinct ventricles and a com- 

 plete double circulation ; finally, they differ from both rep- 

 tiles and birds in having the skin more or less clothed 

 with hair, the lungs freely suspended in a thoracic cavity 

 separated from the abdominal by a muscular partition, the 

 diaphragm, and in the possession by the females of mam- 

 mary glands. In economic uses to man mammals are the 

 most important of all animals. They furnish a great por- 

 tion of the animal food of many human races, likewise a 

 large amount of their clothing. Horses, asses, oxen, 

 camels, reindeer, elephants, and llamas are beasts of burden 



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