FOSSIL ANCESTOKS OF THE HORSE 513 



extra toes in the fore- aiul hind-feet still remain, but they are evidently 

 shrinking in size. The changes in the molar teeth are also very con- 

 siderable. As will be seen on reference to fig. 668, the teeth are passing 

 from the brachydont or short-crowned to the hypsodont or high-crowned 

 variety, a change which goes on progressively in correspondence with the 

 vanishing of the extra digits. In the upper molars of the Hi23parion 

 there is a distinctive feature which is at once recognized by the anatomist, 

 in the presence of an interior column of dentine completely isolated from 

 the rest of the mass, as shown in the section of the upper molar 

 (e, fig. 667) close to the bottom, in the form of a white oval spot 

 surrounded by a double line. 



There can be no doubt that the Hipparion was remarkably like a horse, 

 though possibly not a direct ancestor. It was somewhat smaller than the 

 Wild Mongolian Horse, of which an illustration is given on Plate LXXIII, 

 and differed from it in the presence of the extra digits, which were, 

 nevertheless, becoming rudimentary. The animal evidently used only the 

 single hoof, the extra toes being some distance off the ground surface. 

 It . may be remarked that some of the species of Protohippus are said to 

 have been as large as an ass; this is particularly the case with the 

 European Hipparion. 



Proceeding from the Lower Pliocene to the Upper, the Pliohippus is met 

 with, in which the extra digits have become entirely rudimentary, closely 

 approaching in form the splint bones as they are found now in the limbs 

 of the horse. The lower phalanges and the hoofs of the extra digits which 

 were depicted in the Protohippus have entirely vanished. The ulna and 

 the fibula are very much the same as we find them in the horse, the molar 

 teeth are assuming a more equine character. 



The next step is to the Pleistocene and recent strata in which the fossil 

 remains of the true horse are found. Some of the fossil types have, 

 however, peculiarities of their own, such as the large nasal development 

 of the Hippidium from South America, figured in Plate LXXII. The extra 

 toes, the ulna, and the fibula are now in their present rudimentary form, 

 the molar teeth show the characteristic hypsodont type, and the anterior 

 separate column of dentine has entirely disappeared in the upjoer molars. 

 The history of the evolution of the horse, so far as the evidence furnished 

 by geological researches is available, is thus complete, and surely a more 

 connected and consistent story was never constructed. 



According to promise, the chain of events in the descent of Equus 

 cdballus has been traced along its many links from the most distant, the 

 Eohippus of the Lower Eocene, to the modern horse found in recent 

 geological deposits. 



