196 THE MAMMALIA. 



partition between the nostrils became a firm bony 

 support for the horn, an ossification, which is not 

 unfrequently met with in the tapir, as was shown 

 on Fig. 31. 



America, too, had its family of Ehinoceroses, 

 which seem to have branched off from the Middle 

 Eocene Tapiridae, and comes forward distinctly in 

 the Upper Miocene as Aceratherium. Forms similar 

 to it are found in the Pliocene, but did not leave 

 any descendants to the following period. The 

 causes of its extinction in the New World are not 

 clear. But the reason of the dying out of the 

 Diluvial species in the Old World, or its withdrawal 

 from the temperate zones to tropical regions, seems 

 to be pretty obvious. For even though individual 

 forms such as the rhinoceros with the bony par- 

 tition between the nostrils were, like the mam- 

 moth, able to endure a rougher climate, still they 

 were not able to face the coming of the Glacial 

 period. What prevented them withdrawing before 

 it we certainly do not know ; still we may, at all 

 events, look upon them as having been the victims 

 of climatic changes. Others, which were able to 

 avail themselves of the land bridges for with- 

 drawing southwards, survived. 



An animal of the rhinoceros type, which was 



