6o 



THE ODD-TOED UNGULATES. 



smaller, has the last phalanx pointed, and, 

 there can be no doubt, both it and the first 

 digit, or pollex, which is quite rudimentary, 

 bore a false claw, which could touch the earth 

 only on sinking into soft or marshy ground. 

 Henceforth the first digit disappears entirely. 

 In Orohippus from the Middle and Upper 

 Eocene the fifth still carries a false claw, but 

 in Mesohippus from the Lower, and Miohip- 

 pus from the Upper Miocene, is already re- 

 duced to the metacarpal (or metatarsal) bone, 

 having no phalanges, while the second and 

 fourth digits have become smaller. In Pro- 

 tohippus from the Lower Pliocene, as well as 

 in the succeeding genera, the metacarpal bone 

 of the fifth digit has also vanished, and the 

 second and fourth digits no longer carry hoofs 

 but claws. Then these two digits likewise 

 get reduced to their metacarpal bones in 

 Pliohippus belonging to the Upper Pliocene. 

 The last member of this long series of genera 

 forming successive links in an unbroken chain 

 was a horse which was similar to the domes- 

 ticated horse, but possessed rather differently 

 formed incisors, and which, during the Qua- 

 ternary period, roamed over the whole of 

 America, both North and South, so that it 

 has left remains in the deposits of the Pampas, 

 as well as in the caves of Brazil and the 

 alluvium of the United States. 



In the Old World we have a similar though 

 a less complete series. Our horses appear 

 to be traceable back to the Palaeotherium as 

 their stem-form, this being an equivocal in- 

 termediate type with four toes on the fore- 

 feet, and apparently also the stem from which 

 the rhinoceroses have been derived. But in 

 the genus Anchitherium of the Upper Eocene 

 and the Lower Miocene the equine characters 

 are already expressed with the utmost dis- 

 tinctness in the dentition as well as in the 

 structure of the feet, and may be traced 

 through the genus Hipparion, corresponding, 

 we may say, to the American Protohippus, 

 a genus which had three toes, and whose 



numerous remains, found in Upper Miocene 

 deposits at Pikermi in Greece and Sansans 

 at the foot of the Pyrenees, prove that these 

 elegant animals then traversed southern Eu- 

 rope in numerous herds. 



I cannot enter into details here, but will 

 only state that none of the genera belonging 

 to the series in the evolution of the American 

 horses is identical with any one of those 

 belonging to the succession on this side of the 

 ocean, and that the initial differences are 

 greater than those at the end of the two series. 

 The difference between the Anchitherium of 

 the Lower Miocene of Europe and the 

 Mesohippus on the same horizon in America 

 is considerable, while the differences between 

 the Quaternary horses of the two hemispheres 

 are but slight. The series have accordingly 

 approached one another instead of presenting 

 increasing divergencies. But in both series 

 is seen the same tendency to form out of 

 small, plump, plantigrade or semi-plantigrade 

 animals, omnivorous in their diet, and pro- 

 bably dwellers in marshy districts, larger, 

 slimmer, light-footed herbivora inhabiting dry 

 steppes. 



To sum up, we see in the Perissodactyla a 

 great original and old order which has gradu- 

 ally declined in the process of geological 

 evolution. The stems to which we can now 

 with greater or less probability refer the 

 branches of our present fauna were much 

 more varied, much richer in forms than they 

 are now. There has been a gradual decay 

 along with a one-sided development. By 

 domestication the highly specialized type of 

 the horses has reconquered the domain which 

 it had lost at the beginning of the present 

 geological epoch; the other types, not capable 

 of domestication, seem to be surely advancing 

 towards extinction, in which several pretty 

 rare types have preceded them, types which 

 have gradually died out in the course of 

 evolution, and of which we do not need to 

 speak here. 



