A VIEW IN PERSPECTIVE. 



haps has even been declining. The mammals again 

 give a very pretty illustration of the same principle. 

 This, the highest class of vertebrates, appeared first 

 in the Triassic (6). These early mammals were all 

 small animals and were of the lowest type, the mar- 

 supials. During this and the two subsequent ages, 



FIG. 20. 



Jurassic (7,) and Cretaceous (8), the mammals prob- 

 ably continued to exist with very little change, the 

 few remains found during these ages not certainly 

 indicating any special advance over those of the 

 Triassic (6). At the close of the Cretaceous (8), 

 there seems to have been some influence acting 

 upon the class which started them into a remarkably 

 active development. Just when this impulse oc- 

 curred, we do not know, but with the beginning of 



