vi] DISTRIBUTION OF SELECTED GROUPS 137 



again with Protohippos which suddenly appears in 

 America, together with the Eurasian Hipparion. 

 The latter persisted in Europe into the Upper 

 Pliocene (Britain to Africa, Persia, Russia, China 

 and India) ; with the early Pliocene it also crossed 

 into America, where its various kinds are dis- 

 tinguished as Neohippdrion, and some of these 

 which got right into South America are called 

 Hippidion. All these Hipparions had admittedly too 

 specialised a tooth -pattern to serve as the ancestors 

 of Equus ; their feet were still tridactyle, but the 

 side -toes were slender and only just touched the 

 ground. Here is a gap, usually slurred over. Crea- 

 tures intermediate between these three-toed animals 

 and the genus Equus (in which the side-toes are 

 reduced to splints on either side of the middle or 

 cannon-bone) are unknown. 



The genus Equus appears as E. sivalensis in 

 Pliocene Asia ; other species, e.g. E. stenonis, in 

 Europe, supposed ally of zebras. E. caballm, the 

 horse, appears with the Pleistocene in Europe, North 

 Asia and North-West America from Alaska to 

 California ; and a whole crowd of other horses, 

 amongst them the large pony-sized E. scotti, overran 

 the whole of America, all to die out there with the 

 Pleistocene epoch. But in Asia and in Mediterranean 

 countries appear the asses, tar pan and djiggetai and 

 this dweller of steppes extended in interglacial times 



