50 



THE ORIGIN OF MAN 



seas until the close of the Mezozoic. To under- 

 stand this evolution, it will be necessary to study 

 the organization of these two types of animals. 



Amphibia differ from fishes in that they have 

 legs and not fins, each leg bearing fingers or toes. 

 Nearly all of them breathe by gills when very 

 young and may retain these organs throughout 

 life; lungs are however, nearly always present in 

 the adult. All Amphibia are cold-blooded ani- 

 mals and the name, which means living a double 

 life, was given them because many of them live 

 both on land and in fresh water. The sexes are 

 always separate and in the great majority of 

 species the small eggs are fertilized in the water 

 and develop there without further care from their 

 parents. The development is therefore very 

 much the same as in fishes and very unlike that 

 of the higher vertebrates. The eggs of frogs and 

 toads for example, develop into small animals 

 commonly known as tadpoles or polywogs, with 

 a more or less large and rounded head and body 



