CHAPTER XVIII 

 THE VERTEBRATES {continued): MAMMALS 



The mammals constitute the highest group of animals, 

 including man, the monkeys and apes, the quadrupeds, the 

 bird-like bats and fish-like seals and whales; in all about 

 2500 species. They are found everywhere except on a few 

 small South Sea islands. Only a few species, however, have 

 a world-wide distribution. The. name Mammalia is de- 

 rived from the mammary or milk glands with which the 

 females are provided and by the secretion of which the young 

 of this class, born free in all but a few of the lowest forms, 

 are nourished for some time after birth. 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, which attains a length of a hun- 

 dred 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 complete double cir- 

 culation; finally, they differ from both reptiles 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 mammary glands. In economic uses to 

 man mammals are the most important of all animals. They 

 furnish the greater portion of the animal food of many human 

 races, likewise a large amount of their clothing. Horses, 



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