198 THE MAMMALIA. 



smaller species of bygone days, for instance, the 

 Rhinoceros minutus. 



Before passing on to the most docile and 

 important group of the Odd-hoofed animals, the 

 horses, let us first turn our attention to a few of 

 the American forms, which are distinguished partly 

 by their size and partly by the, in most cases, very 

 unusual form of their skull ; in the struggle for exist- 

 ence these animals, however, neither changed nor 

 left descendants which adapted themselves to cir- 

 cumstances. Their existence reminds us of the 

 Elasmotherium, inasmuch as they neither explain 

 the present (hence in reality stand apart from our 

 subject here), nor do they awaken in us other ideas 

 for understanding the organic world ; but they 

 bear witness to the incredible exuberance, we may 

 almost say the capriciousness, of organic produc- 

 tivity during the Late Tertiary and Diluvial periods 

 while the animals were becoming extinct, and 

 which periods were followed by our Present age, 

 with a certain stability of the inorganic and 

 organic worlds. In this stability of forms, moreover, 

 we see one of the preliminary conditions of the 

 morphological and social development of mankind. 



The lowest strata to the east of the Eocky 

 Mountains contain the remains of the Brontotheridce, 



