﻿lxxii 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. lxxii, 



common ancestors of the groups that were thus destined to 

 flourish were accompanied by other generalized Ungulates which 

 began to acquire running feet without any of the requisite changes 

 in the arrangement of the wrist- and ankle-bones, and often showed 

 a deepening or other complication of the grinding teeth without 

 any infilling of the resultant hollows by cement. These animals 

 with ' inadaptive modifications ' (as Kovalevsky termed them) 

 were soon handicapped in the race with their better-endowed 

 contemporaries, and all became extinct before they had advanced 

 far. In South America, on the other hand, which was an isolated 

 centre of mammalian development during the greater part of the 

 Tertiary Era, all the early Ungulates began to advance in the 

 ' inadaptive ' manner, without any more efficient contemporaries 

 and competitors. Here we are, therefore, able to follow to the end 

 a course of evolution which was abruptly stopped in the Northern 

 Hemisphere. It resulted in less variety than is observable among 

 ordinary Ungulates, but some of the later genera are strange 

 mimics of the Rhinoceroses and Horses in outward shape. At 

 least one of the bulky three-toed forms acquired a rhinoceros-like 

 horn : and some of the small one-toed forms exhibited a greater 

 reduction of the lateral toes even than in the horse. The molar 

 teeth were also sometimes invested with a little cement, but they 

 never became such effective grinders as those of the Horses and 

 Elephants. The latest of these South American animals, indeed, 

 soon disappeared when a land-connexion in the Pliocene Period 

 allowed the animals of North America to invade the Southern 

 Continent and compete with them. 



None of the South American Ungulates developed the sym- 

 « metrical pair of toes such as characterizes so large a proportion of 

 the northern forms, but their single enlarged toe, which became 

 the centre of symmetry, was always the third, as in the ordinary 

 tapirs, rhinoceroses, and horses. It is, therefore, interesting to 

 note that among the Australian Marsupials, which have inde- 

 pendently developed a hind foot for rapid locomotion or leaping 

 on hard ground, the largest toe is not the third, but the fourth. 

 So far, unfortunateby, we lack nearly all the ancestors of these 

 animals ; but available evidence seems to show that they are 

 descended from tree-dwellers in which the first digit had become 

 opposable. When they began to live habitually on the ground 

 and the foot became modified accordingly, this digit dwindled to a 

 projecting rudiment (as preserved in D/prototloii), and the axis of 



