436 THE CHANGING GENERATIONS 



pupal stage. The development of complete metamorphosis was as impor- 

 tant an evolutionary step for the insects as was the development of the 

 shelled egg for land vertebrates, for each of these advances overcame 

 obstacles to full occupancy of terrestrial environments. 



The "stegocephalian" amphibians reached their climax in the Carbon- 

 iferous and early Permian and then rapidly diminished; their last sur- 

 vivors, as we have noted, were small aquatic forms that died out during 

 the Triassic period. In part the decline of these amphibia may have been 

 caused by the increasing aridity and lowered temperature of Permian 

 times; but in greater measure it was probably due to inability to compete 

 with the newly arisen reptiles, whose history is recounted in the next 

 chapter. 



