274 BRITISH MAMMALS 



extinct quite recently {Onohippidium^ for example). Equus caballuSj 

 practically identical with Equus przevalskii^ also seems to have 

 been evolved in Asia ; while forms closely allied to this species 

 migrated to America, and filled the whole of America during the 

 Pleistocene Epoch with horses belonging to the genus Equus. 

 Equus cahallus itself, however, does not seem to have entered 

 North America, but to have confined its range to the whole of 

 Europe and Temperate Asia. The only living wild horse — Equus 

 przevalskii — is scarcely separable specifically from Equus caballuSy 

 and photographs of it may well stand for representations of the 

 kind of horse familiar to our far-distant ancestors as the wild 

 horse of Britain.^ But there also developed from the original 

 horse stock in Asia, other types belonging to the ass and zebra 

 group which are scarcely distinguishable from Caballine horses in 

 their bones or teeth, though they may be very different in 

 outward-aspect, in the markings, the growth of the mane, of the 

 tail, and of certain callosities on the inner side of the hind 

 limbs. 



There is almost every graduation in form between the horse- 

 like wild ass, Equus hemionus (the Kiang) and the magnificent, 

 liberally striped Grevy's zebra of Eastern Africa. The asses and 

 zebras mainly differ from the True Horse (i) in their having 

 a stiff, hog-like mane ; (2) in the absence of a fully plumed tail 

 (that is to say, they have a tail in which the long hairs grow 

 merely in a tuft at the end, and do not start growing from the 

 base of the tail) ; and (3) in the fact that all the zebras and one 

 or two of the asses are striped to a greater or less degree. It 

 has been said that a form of ass, probably identical with that 

 source of the domestic donkey, the wild ass of North-east Africa 

 {Equus taniopus), has been found fossil in England in the 

 Pleistocene formations, but this statement is of doubtful value, 

 and von Zittel equally doubts the existence of this animal (in a 

 wild state) in Central Europe. There is so little to distinguish 

 the African wild ass from the horse in its teeth, or even in the 



^ The adult males of this Prjevalski's horse in Woburn Park are singularly 

 like underbred cart-horses. 



