CHAPTER XXVI. 

 CHORDATA (CONTINUED): CLASS MAMMALIA. 



338. General Statement. The mammals are, on the 

 whole, the highest vertebrates. Here we include forms 

 as widely different as the kangaroo, the whale, the seals, 

 the horse, the lion, the bats, the monkeys, and man. It 

 will be seen that they differ more among themselves than 

 the birds do, and yet they agree in some quite interesting 

 facts. They all develop certain glands which in the 

 female produce milk. These (mammary) glands give 

 the name to the class. Instead of feathers, the skin 

 develops hair. There may not be much, as in the whales, 

 but even in these a certain amount is found, especially 

 in the younger stages. 



The mammals are like the birds in being air breathers, 

 warm-blooded, and in having a four-chambered heart. 

 The birds are more highly specialized in some respects 

 and the mammals in others. 



The lowest types of mammals, the monotremes, are 

 like the birds in the fact that they lay large eggs and the 

 young are hatched from these. But all the other mam- 

 mals produce small eggs which are partly developed in a 

 special organ (uterus') of the mother; and when the young 

 are brought forth they are already similar in form and 

 structure to the adult, though they are not able to take 

 care of themselves. In the marsupials, the second divi- 

 sion, the young are very immature and are placed in a 

 pouch (marsupium) on the outside of the body in the 

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