EVOLUTION OF THE VERTEBRATES 



151 



the persistence of abundant wet regions were ideal conditions for 

 amphibian evolution. It is recorded of the earliest transitional 

 types that they retained both lungs and gills, and had both limbs 

 and a tail fin. During the Carboniferous they attained a consider- 

 able variety of forms, but the Permian brought an extension of 

 continental areas, with relatively dry surfaces and seasonal 



-=:.^:=5^.j=z^ 



O F 



Fig. 84. — Group of extinct Amphibia. A, B, D and E from the Carboniferous; 

 C and F from the Permo-Carboniferous. A, Pylonius; B, Ampliihamus; 

 C, Cacops; D, Cricotus; E, Diplocaulus; F, Erynps. (From Newman, after 

 Osborn, based on restorations by Gregory and Deckert.) 



changes in the bodies of fresh water, which were unfavorable to 

 amphibian life. They have been able to persist, but beset by the 

 limitations of two environments and the narrowness of a transi- 

 tional life zone they have been unable to rise above a corresponding 

 limitation of forms. 



During their ascendency some time in the early Carboniferous 

 the Amphibia developed a number of terrestrial forms which were 

 made possible by the altc^rnation of arid seasons with periods when 

 the streams were full to overflowing and herbaceous flood-plain 



