322 GEOLOGY. 



tivc ungulate form; monkeys of the South American type had evolved 

 probably from a North American Eocene lemuroid, while rodents of 

 the porcupine type, but not of other orders, had been derived from 

 some unknown immigrant form. That the connection was only par- 

 tial or very temporary, seems to be implied by the absence of most 

 of the great North American groups, such as the creodonts, carnivores, 

 condylarths, artiodactyls, perissodactyls, and insectivores. The absence 

 of proboscidians implies a lack of connection between South America 

 and Africa, where these forms had been developing during the Eocene 

 and Miocene. Many carnivorous and herbivorous marsupials closely 

 similar to those of Australia lived during this interval in South America, 

 implying either connection in that direction, or pronounced parallel 

 evolution. If the former, it is unknown whether the migration was 

 toward or from South America. This remarkable South American 

 fauna is a striking instance of evolution on a large scale in comparative 

 isolation, and in relative freedom from the severe stimulus of effective 

 competition, powerful carnivores, and shifting geographic relations. 1 



On the opening of connection between the two Americas in Plio- 

 cene times, the faunas of each division invaded the other. Horses, 

 mastodons, deer, carnivores of the dog and cat families, llamas, and 

 tapirs from the north invaded South America, while certain gigantic 

 sloths {Megatherium, Mylodon, Megalonyx, and Glyptodori) invaded 

 North America. The latter group did not maintain themselves in 

 North America beyond the Pleistocene period, whether because of 

 the physical environment, the ice invasions, or the struggle with a 

 superior fauna, cannot be affirmed. The northern invaders were more 

 successful in South America though not conspicuously so, as only a 

 portion of them have living descendants there. 



That the extraordinary evolution of the unclomesticated placentals 

 experienced a decline at the close of this period was a natural result 

 of the glacial invasions that followed, and of the even more potent 

 influence of man. 



During the period, the evolution of the mammals pursued essentially 

 the same lines as before. The herbivores continued to occupy the fore- 

 most, as well as the fundamental place. Both the cdd- and even-toed 

 ungulates completed their deployment into all their present families, 



1 For late data see the Reports of the Princeton University expedition to Pata- 

 gonia, 1896-99. 



