x EVOLUTION OF THE HORSE 247 



the molar teeth is, too, a little different from that of Eqwus. Proto- 

 hi r ppus of the North American Pliocene is also three-toed, but the 

 two additionally-developed toes are smaller than in Hipparion. 

 Other forms are dealt with below in connexion with the ancestry 

 of Perissodactyles. It is a curious fact about Hipparion, which 

 is not now regarded as on the direct line of equine descent, that 

 the edges of the enamel plates of the molars may show a com- 

 plicated folding very like that presented by that clearly terminal 

 form of Perissodactyle life, the gigantic Elasmotlierium. This is 

 indicative of higli specialisation, which ended in extinction. 



Ancestry of the Horses. The Lophiodontidae and the 

 Palaeotheriidae are two of the most interesting extinct families 

 of Perissodactyles ; for among them we find what would appear 

 to be the ancestral forms of both the existing Tapirs and Horses. 

 The Ehinoceroses also would seem to be derivable from the 

 Palaeotheriidae. The very vagueness of the characters of these 

 creatures, considered from a classificatory point of view, has led to 

 much diversity in their placing. This though gratifying to the 

 evolutionist is tiresome to the writer who wishes to give a 

 methodical account of their various characters. It will be best 

 perhaps not to attempt an accurate placing or to reconcile con- 

 flicting opinion, bub to give some salient features of osteology 

 which lead to the belief in their relationship to existing groups of 

 Perissodactyles. A book upon the history of mammals would be 

 incomplete without some account of that well- ascertained series of 

 forms which seem to connect these primitive Perissodactyles with 

 the modern Horse. Equus, in fact, is not only the " show horse " 

 of the doctrine of evolution, but also the " stalking horse." 



In the Eocene of both Europe and America are met with a 

 number of forms from which we may start. Hyracotherium, 

 regarded on the one hand as the type of a sub-family of the 

 Equidae themselves, and on the other as a member of the family 

 Lophiodontidae, was a small -sized animal, three feet or so in 

 length ; it possesses the complete Eutheriau dentition with a slight 

 diastema. The orbits are not separated from the temporal fossa ; 

 the fore-limbs were, four-toed, the hind three-toed, with moderately 

 long metapodia, especially on the hind-feet. The shoulder blade 



Soc. 1900, p. 379) in the cave which produced the remains of Glossotherium. A 

 piece of skin covered with Fox-red hair, possibly spotted with paler areas, is believed 

 to be a relic of Onohippidium. 



