Page:Popular Science Monthly Volume 12.djvu/698

678 extinct, leaving apparently no successor, unless possibly we have in the Proboscidians their much-modified descendants. Their genetic connection with the Coryphodonts is much more probable, in view of what we now know of the two groups.

Besides these peculiar mammals, which are extinct, and mainly of interest to the biologist, there were others in the early Tertiary which remind us of those at present living around us. When a student in Germany some twelve years ago, I heard a world-renowned Professor of Zoölogy gravely inform his pupils that the horse was a gift of the Old World to the New, and was entirely unknown in America until introduced by the Spaniards. After the lecture I asked him whether no earlier remains of horses had been found on this continent, and was told in reply that the reports to that effect were too unsatisfactory to be presented as facts in science. This remark led me, on my return, to examine the subject myself, and I have since unearthed, with my own hands, not less than thirty distinct species of the horse tribe in the Tertiary deposits of the West alone; and it is now, I think, generally admitted that America is, after all, the true home of the horse.

I can offer you no better, illustration than this of the advance vertebrate paleontology has made during the last decade, or of the important contributions to this progress which our Rocky Mountain region has supplied.

The oldest representative of the horse, at present known, is the diminutive Eohippus, from the lower Eocene. Several species have been found, all about the size of a fox. Like most of the early mammals, these Ungulates had forty-four teeth, the molars with short crowns, and quite distinct in form from the premolars. The ulna and the fibula were entire and distinct, and there were four well-developed toes, and a rudiment of another on the fore-feet, and three toes behind. In the structure of the feet, and in the teeth, the Eohippus indicates unmistakably that the direct ancestral line to the modern horse has already separated from the other Perissodactyles. In the next higher division of the Eocene, another genus, Orohippus, makes its appearance, replacing Eohippus, and showing a greater, although still distant, resemblance to the Equine type. The rudimentary first digit of the fore-foot 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 in the same horizon with Dinoceras, and others lived during the upper Eocene with Diplacodon, but none later.

Near the base of the Miocene, in the Brontotherium beds, we find a third closely-allied genus, Mesohippus, which is about as large as a