384 PROGRESSIVE DEVELOPMENT 



a nourishing fluid for their young in certain glands of their skin. 

 Their rate of metabolism was much higher than the reptiles. 



Am. Museum Nat. History 



The Paleozoic period is noted for its luxuriance of plant growth. Ferns, club mosses, and 

 ferns related to the horsetails attained the size of trees and formed dense forests. Many 

 cone-bearing trees are found, but none of the higher types of flowering plants. As the plants 

 died and became part of the swamps, some unusual tremendous pressure transformed them 

 into coal beds. 



Hence they are called warm-blooded. They retained the eggs 

 inside their bodies until the young were almost fully developed, 

 and supplied them with nourishment during development. These 

 were the mammals. The platypus or duckbill of Australia is 

 thought to be a transitional form between the egg-laying birds and 

 the mammals. It is furry, has a bill like a duck, lays eggs, and 

 feeds, its young with milk (page 513). The marsupials, such as 

 the kangaroo, are somewhat transitional. The young are born 

 before fully developed. They climb into a pouch in the abdo- 

 men of the mother and complete their development there. From 

 the early mammals of the Cenozoic era have developed the many 

 species which dominate the earth to-day. Man belongs in this 

 great group of mammals. 



