XIII 
THE GEOLOGICAL EVIDENCES OF EVOLUTION 
387 
ably indicates that the direct ancestral line to the modern 
horse has already separated from the other perissodactyles, 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. Orohippus 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 
