274 THE DOCTRINE OF DESCENT. 
abundant fossil material. In the older Tertiary strata 
we encounter the remains of two Ungulate families, 
the Paleotheride and the Anoplotheridz, 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 Palezotheride 
includes, among the present Ungulata, the horse, tapir, 
, and rhinoceros. The transi- 
i tion from the Palzotherium 
'' to the horse may be directl 
y y 
traced, and this, moreover, 
in the two most important 
characters, the dentition and 
ry the feet. In the Anchithe 
rium and Hipparion, the 
transformation from the tri- 
dactyle to the unidactyle 
i : a Ungulate is accomplished ; 
sg eee 
FIG, 26. Skeleton of the foot. (P) Anchithe- and Riitimeyer’s brilliant a 
rium. (H) Hipparion, (E) Horse. "searches have shown how, in 
the milk dentition of each genus, the definitive dentition 
