A BIG FOSSIL BONE. 77 



dominant class ; and, when another class out- 

 strips it in development, the new dominant 

 kinds are apt to live down the bigger species 

 among their predecessors. Thus, in the 

 very earliest times, the molluscs were appar- 

 ently the dominant class ; and very big cuttle- 

 fish and other cephalopods were frequent 

 though none of them, perhaps, were quite so 

 big as our own gigantic squids. At a later 

 date, the reptiles were developed, and grew 

 to be the leading race on earth ; and during 

 that period the bigger saurians attained to 

 extremely large dimensions. Ever since the 

 opening of the tertiary period, however, the 

 mammals have become the forefront of the 

 animal series, and big mammals have every- 

 where replaced big reptiles. But there were 

 some few insulated spots where mammals did 

 not penetrate for a long time, and here birds 

 were the leading class. In such cases terres- 

 trial birds grew to be very large indeed. 



On this simple principle we can account 

 for almost all the big creatures now existing 



