HOKSES. 371 



and is probably identical with the form seen by Speke and Grant 

 during their journey to the lake regions. 



Of the fossil forms of Equus there have been thus far described 

 some twenty or more species, which date back in both hemi- 

 spheres to the Pliocene period. Only two of the recent species are 

 positively known in the fossil state — the E. asinus, or ass, which is 

 Post-Pliocene, and the horse (E. caballus), which appears to be 

 first known from the Upper Pliocene ; it is not improbable, how- 

 ever, that some of the Post- Pliocene equine remains of Central 

 Europe belong to the dziggetai. The Pliocene species seemingly 

 most nearly related to the modern horse are the E. major or E. 

 Americanus, from the deposits of North America, and the E. Ste- 

 nonis, from the Val d'Arno, Italy. The only other genus of 

 Equidse besides Equus is Hippidium (Pliohippus ?), which occurs 

 fossil in the Pliocene deposits of both North and South America. 



Nehring, from a careful study of the numerous fossil remains of 

 horses found in Germany and elsewhere, arrives at the conclusion 

 that the present European animal — at least, as representing some 

 of the races — instead of being, as is commonly supposed, a recent 

 introduction from Asia, is in reality indigenous to the region which 

 it now inhabits in a domesticated state, and that it has been a con- 

 tinuous inhabitant of Central Europe, then largely in the form of 

 a steppe country, supporting a steppe fauna, ever since the early 

 part of the Quaternary epoch. 



Extinct Animals related to the Horse and Tapir.— In no 

 group of mammals, probably, is the difficulty of drawing family 

 boundaries greater than among the perissodactyl ungulates, a cir- 

 cumstance due chiefly to the perfection with which many of the 

 lines of descent have been traced out, and to the intimate rela- 

 tionship which the animals of one line bear to the animals of one 

 or more other lines. Thus the primitive ancestors of the horse — 

 the four-toed and three-toed (fore and aft) Eohippus and Orohippus 

 (Hyracotherium) from the Eocene, the three-toed Mesohippus and 

 Miohippus (Anchitherium) from the Miocene, and the Pliocene 

 Hipparion (also Miocene) and Protohippus— which form an almost 

 continuous chain connecting Eohippus and the Equidse, belong to 

 two or three families, Lophiodontidse, Palaeotheridse,* Anchithe- 



* To the typo genus of this family, the Eocene Pateotherium, many paleon- 

 tologists have traced the ancestral line of the European horse— Equus StenonU, 



