148 THE MAMMALIA 



nected only with the drying up of marshy lands. 

 And if, by some extravagant flight of the imagina- 

 tion, we could conceive the existence of a one-toed 

 leviathan, the very fact of its possessing a one-toed 

 foot would be the cause of its speedy extinction. 

 As regards dentition also the hippopotamus shows 

 signs of being geologically very old. The skull of 

 the unwieldy creature reminds one of a clumsily- 

 formed box. The breadth and height of the muzzle 

 are produced by the enormous development of the 

 middle incisors and of the canines. All of these 

 teeth are furnished with roots that are not closed, 

 but open wide apart. It is certainly not impossible 

 that these teeth assumed this form first among the 

 nearer ancestors of the river-horse. But it is more 

 probable still that the disposition to assume this 

 form was a remote inheritance, and that it was 

 only by accommodating itself to feed on aquatic 

 plants that, as regards position and size, the teeth 

 have grown into such ugly but useful tusks. 



It has been already stated that the river-horse 

 is the only representative of its family. This 

 remark requires some explanation ; for we have 

 not only the well-known and so-called Nile-horse, 

 which is distributed over a large portion of central 

 Africa, but there is a second species, only 5 feet 



