Il6 THE LIVING WORLD. 



age that the plant world received a similar impulse, 

 causing it to expand into modern forms. 



The second date of importance in the history of 

 animals was at the beginning of the Tertiary (9). 

 With this age we may say that the strictly modern 

 order of nature began. From this time we begin 

 to find in abundance the families and genera of ani- 

 mals which characterize the world to-day. Of our 

 existing species of animals, the lower ones were the 

 first to appear. Or stated in other terms, the lower 

 species of animals are of longer duration than the 

 higher ones. In the lowest rocks of the Tertiary 

 (Eocene), we find representatives of existing reptiles 

 living contemporaneously with a world of extinct 

 mammals. None of our living mammals had then 

 appeared, though modern reptiles were abundant. 

 Of the mammals, too, the lower orders approached 

 the modern condition first, the Insectivora and the 

 Edentata considerably antedating the Primates. 

 During the Tertiary, the approximation toward 

 modern fauna was very rapid. Old species disap- 

 peared, and new ones appeared leading directly up 

 to the present time. The new era inaugurated by 

 the Tertiary was not, however, so marked as that 

 coming in with the Mesozoic, for except in the class 

 of mammals there was far less expansion of type 

 than occurred in the early Mesozoic. Still the new 

 era is sufficiently well marked. Of the causes which 

 produced it we have no satisfactory knowledge. 



I 



A Constant Change of Species. 



Looking over the whole history as seen by our 

 collections of fossils to-day, we find that through all 



