190 DINOC'ERATA. 



clearly the monarclis of the region iu which they lived. In tlie upper 

 Eocene, Diplacodon, about the size of the rhinoceros, was the largest 

 mammal, but each of these thi'ee died out in the period in which it 

 flourished. 



At the base of the Miocene, the huge BrontotheridcB, nearly as large 

 as the elephant, suddenly appear in great numbers. They remained for 

 a short time the dominant land animals, and then became extinct. 



The Proboscidians were the giants of the Pliocene, and hold the 

 supremacy in size to-day, but are evidently a declining race, and must 

 soon disappear. 



The cause of the successive disappearance of each group of these 

 large Tertiary mammals is not diflicult to find. The small brain, highly 

 specialized characters, and huge bulk, rendered them incapable of 

 adapting themselves to new conditions, and a change of surroundings 

 brought extinction. Smaller mammals, with larger brains, and more plastic 

 structure, readily adapt themselves to their environment, and sm-vive, or 

 even send off new and vigorous lines. 



The Dinocerata, with their very diminutive brain, fixed characters, and 

 massive frames, flourished as long as the conditions were especially 

 favorable, but, with the first geological change, they perished, and left 

 no descendants. 



Classification of Dinocerata. 



The relations of the Dinocerata to other orders of mammals have now 

 been fully considered in the preceding pages, and the main conclusions 

 reached are given on pages 173 and 177. The generic sub-divisions of 

 this group do not appear widely separated from each other, although at 

 least three types, from successive geological horizons, can be distinguished. 

 The species are numerous, and well marked, and there is strong evidence 

 that many, if not all of them are from separate horizons. The differences 

 due to age and sex are also manifest, and have been duly considered in 

 estimating distinctive characters. 



