144 THE MAMMALIA. 



land, and during the Later Tertiary as well. An 

 exchange, therefore, may have taken place, and 

 certainly must have taken place up to the Eocene 

 period, as is proved by the occurrence of Coryphodon 

 in Europe and America, and of Palaeotherium and 

 Anoplotherium in Europe and South America. But 

 for a long period, during the Miocene, there was no 

 connection either between North and South America 

 or between America and the Old World. Hence 

 during this period there must, in any case, have 

 been a further parallel development upon the same 

 basis, a parallel development of Pair-hoofed animals 

 with tuberculate teeth, where the reduction of the 

 toes, discussed above, was an advantage ; and the 

 supposition of an even wider case of convergence is 

 perfectly admissible from a scientific point of view. 



2. THE HIPPOPOTAMUS, OK RIVER-HORSE. 



The Hippopotamus has to be traced back to a 

 similar primary form, for it is the only living 

 representative of the Hoofed animals with tuber- 

 culate teeth which has preserved the old structure 

 of the limbs pretty well unchanged. The still 

 uncut, rootless cheek-tooth (Fig. 18) somewhat 

 resembles a double mitre with a basal setting, which 

 in front and at the back passes over into a three- 



