411 



CHAPTER XXVII. 



CLASSFICATION OF THE EQUID^. 



General Classification — Dififerences between Asses and Horses — Differences be- 

 tween Asses and Zebras — Classification of Asses — Classification of Zebras — 

 Classification of Horses. 



General Classification. — Owing to the extreme want 

 of uniformity in the gaps left, during the process of 

 evolution, between descendants from similar ancestors, 

 we are unable to lay down any exact general rules for 

 classification. The inclusion of horses, asses and zebras in 

 the genus Equus admits of no controversy, because they 

 are the only possessors of the distinguishing characteristic 

 of having only one complete hoofed toe on each foot. 



In the division of a genus into species, we are met 

 with the difficulty that it is impossible to precisely define 

 the meaning of the word, species ; but, for the sake 

 of convenience, we may regard a species as a group of 

 animals which resemble each other, and which can produce 

 fertile offspring more or less similar to themselves or to 

 their ancestors. Buffon's remark that fecundity "is the 

 only characteristic which differentiates or identifies species " 

 is not of general application,* although it serves our 

 purpose in the case of horses and asses, by enabling us 

 to place these respective animals in two different species. 

 Up to the present, experimental research has not solved 

 the question as to the fertility of offspring which could 



* Although the mule and hinny are sterile, we find that the offspring of some parents 

 (such as the ox and bison, and also the dog and wolf) which are much more dissimilar than 

 the horse and ass, are fertile. The dog is supposed to be descended, partly from the wolf, 

 and partly from the jackal. 



