THEIR POSITION IN THE ANIMAL KINGDOM. 7 



between the Mammalia and the rest of the verte- 

 brates has scarcely been looked for anywhere, 

 except among the birds, owing to the obviously 

 high degree of mental development attained by 

 certain members of this class, owing to the stronger 

 and more perfect circulation of their blood. But 

 neither Buffon or any other later comparative 

 anatomist has undertaken to give any specific proof 

 of this link, because, as was said above, other 

 circumstances pointed to a relationship with the 

 Amphibians. 



It will not be necessary to make other vain 

 attempts to bridge over the gap between the 

 Mammalia and the other vertebrates of the present 

 day. It will even be shown that this difficulty is 

 not at all or but little removed by our present 

 knowledge of primeval times. 



More striking still are the numerous isolated 

 forms within the class of the Mammalia themselves. 

 The best known example of this kind of an isolated 

 form of mammal is the horse and its relatives, the 

 genus Equus. The descriptive zoologist places it by 

 the side of the two-hoofed animals. Yet the differ- 

 ence between the one-toed horse and the two-toed 

 oxen and stags remains completely unexplained. 

 Besides this, the more perfect dentition of the horse 



