XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 387 



ably indicates that the direct ancestral line to the modern 

 horse has already separated from the other perissodactylcs, or 

 odd-toed ungulates. 



" In the next higher division of the Eocene another genus, 

 Orohippus, makes its appearance, replacing Eohippus, and 

 showing a greater, though still distant, resemblance to the 

 equine type. The rudimentary first digit of the forefoot has 

 disappeared, and the last premolar has gone over to the molar 

 series, Orohijipus was but little larger than Eohippus, and 

 in most other respects very similar. Several species have 

 been found, but none occur later than the Upper Eocene. 



" Near the base of the Miocene, we find a third closely allied 

 genus, Mesohippus, which is about as large as a sheep, and one 

 stage nearer the horse. There are only three toes and a 

 rudimentary splint on the forefeet, and three toes behind. 

 Two of the premolar teeth are quite like the molars. The 

 ulna is no longer distinct or the fibula entire, and other 

 characters show clearly that the transition is advancing. 



" In the Upper Miocene Mesohippus is not found, but in its 

 place a fourth form, Miohippus, continues the line. This 

 genus is near the Anchitherium of Europe, but presents 

 several important differences. The three toes in each foot 

 are more nearly of a size, and a rudiment of the fifth meta- 

 carpal bone is retained. All the known species of this genus 

 are larger than those of Mesohippus, and none of them pass 

 above the Miocene formation. 



" The genus Protohippus of the Lower Pliocene is yet more 

 equine, and some of its species equalled the ass in size. There 

 are still three toes on each foot, but only the middle one, 

 corresponding to the single toe of the horse, comes to the 

 ground. This genus resembles most nearly the Hipparion of 

 Europe. 



" In the Pliocene we have the last stage of the series before 

 reaching the horse, in the genus Pliohippus, which has lost 

 the small hooflets, and in other respects is very equine. 

 Only in the Upper Pliocene does the true Equus appear and 

 complete the genealogy of the horse, which in the Post-Tertiary 

 roamed over the whole of North and South America, and soon 

 after became extinct. This occurred long before the dis- 

 covery of the continent by Europeans, and no satisfactory 



