2l8 



THE CHAIN OF LIFE. 



of the modern horse. A very similar genus {Orohippus)^ 

 lacking only the fifth rudimentary toe, replaces Eohippus in 

 the Middle Eocene. Mesohippus of the Lower Miocene is as 

 large as a sheep, and has only three toes on the fore-foot and 

 a splint bone, while its teeth assume a more equine character 

 (Fig. 17b). In the Upper Miocene Miohippus continues the 

 line, while Protohippus of the Lower Pliocene is still more 

 equine and as large as an ass, and corresponds with the 

 European Hipparion in having the middle toe of each foot 

 alone long enough to reach the ground. In the Upper 

 Pliocene true horses appear with only a single toe, and splint 

 bones instead of the others. In America, though the horse was 



Fig. 176. — Series of Equine Feet.— After Marsh. 



a, Orohifipus, Eocene, b, Miohippus, Miocene, c, Protohippus, Lower Pliocene. 

 d, Pliohippus, Upper Pliocene, e, Equus, Post- Pliocene and Modern. 



unknown at the time of the discovery of the continent, several 

 species occur in the Tertiary and Post-Pliocene, showing that 

 the genus existed there up to a comparatively late period ; and 

 when re-introduced it has thriven and run wild in the more 

 temperate regions. What cause could have led to its extinction 

 in Post-Glacial times is as yet a mystery. This genealogy of 

 the horse, independently of its evolutionist application, is very 

 interesting. It shows that some Eocene types were suited to 

 continuance, and even adapted for extension, while others were 

 destined to become altogether extinct at an early date. It 

 shows farther that the power of continuance resided not so 

 much in the gigantic and prominent species as in smaller 



