420 CLASSIFICATION OF THE EQUIDJS. 



and the Asiatic Wild Ass ; and divide the former into 

 two varieties (the Nubian Wild Ass and the Somaliland 

 Wild Ass) ; and the latter, into three varieties (Onager, 

 Kiang and Syrian Wild Ass). Domesticity for many thou- 

 sands of years has produced so little effect in altering the 

 distinctive markings of the Nubian Wild Ass, in the case 

 of its domestic relative (the common donkey), that it is 

 reasonable to regard the Nubian Wild Ass and the Somali- 

 land Wild Ass as two separate species. The Onager, 

 Kiang and Syrian Wild Ass do not, respectively, possess 

 characteristics sufficiently well-marked or sufficiently per- 

 manent to warrant their being placed in separate species. 

 We may therefore class them as varieties of one species 

 (Equus hemionus). 



Classification of Zebras. — Naturahsts generally 

 divide zebras into three species, namely, the Mountain 

 zebra, Burchell's zebra and Grevy's zebra ; and class 

 Chapman's zebra. Grant's zebra and Crawshay's zebra as 

 varieties of Burchell's zebra. 



Classification of Horses. — The group of horses 

 which presents the best marked differences from other 

 horses, is Prjevalsky's horse. The characteristics of this 

 wild Mongolian animal (p. 640 et seq.) are distinctive 

 enough to place him in, at least, a separate variety. If 

 it is found that offspring produced by intercrossing with 

 Prjevalsky's horse and the domestic horse are sterile, we 

 shall naturally regard them as hybrids, and shall class 

 Prjevalsky's horse ' as a separate species. Although this 

 wild animal is usually regarded as intermediate between 

 the horse and ass, he is certainly more closely allied to 

 the former than to the latter. The idea entertained by 

 some zoologists that Prjevalsky's horse is a hybrid pro- 

 duced by crossing the kiang with Mongolian mares, has 

 been conclusively refuted by Professor Ewart, who bred 

 on his experimental farm near Penicuik, several hybrids 

 of this kind, all of which were essentially different from 



