236 GEOLOGY. 



forms lived both in Europe and America, and the evolution seems to 

 have gone forward along much the same lines in both countries; but 

 how far this implies free intermigration and how far parallel evolution 

 is a mooted point. 



The rhinoceros family appears in the record a little later than the 

 tapirs and horses, and, although recognized in the later part of this 

 period, had its development chiefly in the next. 



A notable side branch of the tapir-horse-rhinoceros stem appeared 

 in the later part of the period in the form of the titanotheres, which, in 

 the next period, reached titanic dimensions and then soon became extinct. 



The deployment of the artiodactyls. — The even-toed division 

 emerged from the generalized type more slowly. Of the four present 

 groups, Pecora (cattle, sheep, deer), Suina (pigs, peccaries, hippopota- 

 muses), Tylopoda (camels, llamas), Tragulina (chevro tains), the second 

 was represented in the Bridger epoch by a primitive hog (Homacodon) 

 which was much smaller than the modern hog, and had strong canine 

 teeth of somewhat carnivorous aspect. Strangely enough, the ancestral 

 camels seem to have developed on the American continent in the mid- 

 dle and later Eocene, and to have flourished here until the Pliocene, 

 when, having previously sent a branch to South America to evolve into 

 llamas and vicunas, and another into the Old World to become the 

 present camels, they died out in their primitive home. The forerunners 

 of the ruminants appeared in a group of partially differentiated forms 

 (Ccenotheridce and Xiphontidce) , and there was also a rather notable 

 group of small artiodactyls, the oreodons, that seem to have left no 

 descendants. 



Amid all these changes in the more progressive branches of the con- 

 dylarths and their descendants, the primitive type of condylarths lived 

 on with minor modifications, but after the earliest Eocene, it became 

 markedly inferior to its own more progressive kin. 



The development of the carnivores. — As already noted, the ances- 

 tors of the carnivores, the creoclonts, were not sharply distinguished 

 from the primitive ungulates, the condylarths. It has been thought 

 by some paleontologists that the creodonts were the more primitive 

 stem, and that the condylarths diverged from them, as also the eden- 

 tate and rodent branches. This would give the creodonts the central 

 position among the primitive mammals. It has been suggested that 

 they themselves may have branched off at an earlier date from some very 



