HISTORY OF THE EARTH 301 



(6) Amphibians. 



This group, containing frogs, salamanders, etc., 

 is intermediate between fishes and reptiles. As 

 the name indicates, they live partly in water and 

 partly in air. The young, called tadpoles if frogs, 

 breathe with gills and are otherwise much like 

 fishes ; but before they are fully grown they usually 

 develop lungs, shed their fins and gills, and change 

 their habits accordingly. Some amphibians, how- 

 ever, never relinquish their aquatic habits. 



(c) Reptiles. 



The snakes, crocodiles, lizards, turtles, and still 

 other forms now extinct, may at first glance seem 

 to be like the amphibians, but in reality they are 

 very different. They breathe air exclusively and 

 their skeletons are usually rather solid and well 

 constructed. They were formerly far more abun- 

 dant and important in the world than they are now. 



(d) Birds. 



The feathered vertebrates are so plainly set off 

 from all others as to need little description. In 

 spite of the dissimilarity of the two groups, it is 

 known that the birds are much more closely related 

 to the reptiles than to any other class (p. 413). 



(e) Mammals. 



The common quadrupeds, such as cattle, bears, 

 mice, bats, and kangaroos, which as a rule bring 

 forth live young and nourish them with milk, are 

 now the leaders of the organic world. Most of 

 them are clothed with hair and inhabit the land, 

 although some, like the whales, are bare and live 

 in the water. As a group the mammals excel all 

 other animals in intelligence and consequently in 

 power, for in all the history of life, cunning, skill, 

 and resourcefulness have been of more avail in 

 the combat for existence than mere strength or size. 

 B. & B. GEOL. 17 



