442 HISTORICAL GEOLOGY 



LIFE OF THE TERTIARY PERIOD 



Modern aspect of the lower forms of life. Before the 

 Tertiary period, all the important types of plants had made 

 their appearance, and the flowering group had taken the 

 place it now holds in the lead. The lower groups of animals 

 had likewise become much like those we have to-day. The 

 trilobites, brachiopods, ammonites, and other ancient divi- 

 sions had given way to modern groups of crustaceans, bi- 

 valves, cuttlefish, and others. The fishes, amphibians, and 

 reptiles had passed their prime and were represented in the 

 Tertiary period only by species resembling those now 

 living. 



Only the birds and mammals, then, claim our interest, 

 because they alone are still progressing. Of these, the 

 mammals are much the more important, and have left us the 

 better-preserved fossils. They are now the highest and most 

 powerful of the animal groups. 



Generalized mammals of the Eocene epoch. Among 

 the beds of sand and clay which were laid down in the broad 

 Eocene valleys of our western mountain region and certain 

 other parts of the world, abundant skeletons of mammals 

 have been found. They show that many kinds were even 

 then in existence, that they differed considerably in their 

 habits of life, and that they were already the leading ani- 

 mals of their time. At the present day we have no difficulty 

 in distinguishing the several large groups of mammals from 

 each other. Thus we have the flesh eaters (Carnivores), 

 such as the tiger, bear, and wolf ; the hoofed mammals (Un- 

 gulates), such as the horse, buffalo, and deer; the gnawers 

 (Rodents), such as the squirrel and rat; the whales and 

 dolphins (Cetaceans), which are swimmers exclusively; and 

 still others. It is difficult, however, to place the early Ter- 

 tiary mammals in these familiar divisions. Instead, we find 

 varieties which seem to have combined the characteristics of 

 several later groups. For example, it is possible to trace 



