Page:Darwinism by Alfred Wallace 1889.djvu/411

 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 metacarpal 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 discovery of the continent by Europeans, and no satisfactory