274 



THE DOCTRINE OF DESCENT. 



abundant fossil material. In the older Tertiary strata 

 we encounter the remains of two Ungulate families, 

 the Palaeotheridai and the Anoplotheridse, essentially 

 distinguished from, one another by their dentition, 

 and forming the starting-points of the groups of Un- 

 gulates of which some now appear so greatly isolated. 

 The root to which these two families lead back is un- 

 known ; on the other hand, partly from the direct 

 comparison of these genera with the present Ungulata, 

 partly from numerous intermediate links found in the 

 Miocene, Pliocene, and Diluvium, it appears that, in the 

 lapse of time, the separation which characterizes the 

 present age was initiated, and the seeming Isolation was 

 produced by the extinction of the intermediate links. It 

 was this isolation which induced the older systematizers 

 to institute three orders of Ungulata. 



The special pedigree emanating from the Palaeotheridae 

 includes, among the present Ungulata, the horse, tapir, 

 r^ and rhinoceros. The transi- 

 \ j tion from the Palaeotherium 

 / to the horse may be directly 

 traced, and this, moreover, 

 in the two most important 

 characters, the dentition and 

 the feet. In the Anchithe- 

 rium and Hipparion, the 

 transformation from the tri- 

 dactyle to the unidactyle 

 Ungulate is accomplished ; 

 _ and Riitimeyer's brilliant re- 

 searches have shown how, in 

 the milk dentition of each genus, the definitive dentition 



Fig. 26. Skeleton of the foot, 

 rium. (H) Hipparion. (E) Horse 



