350 GENERAL ZOOLOGY 



abundant, progress had been made over preceding periods 

 in the development of backboned creatures adapted to 

 breathing air. These were amphibians, and from the prev- 

 alence of species of this order the period is often called the 

 Age of Amphibians. There were snake-like forms without 

 limbs, and forms with every degree of limb development. 

 Some species grew to be as large as alligators of the present 

 time. 



This age when the amphibians represented the highest 

 development of life upon the earth came to a close about 

 the time that the Appalachian Mountains were being 

 formed. Up to the close of this age the eastern portion of 

 what is now the United States had undergone frequent 

 changes of level. Part of the time the land areas stood well 

 above the water, but the land was frequently overflowed by 

 the sea. With the formation of the Appalachian Moun- 

 tains, conditions became more permanently suited for land 

 animals. In the following ages new forms of life began to 

 come into existence. 



