LIFE 609 



ning of the present divergence between Old and New World types; 

 and (3) the culmination and perhaps initial decline of the mammals, 

 except those domestic species protected by man. 



Tin- Intel-migrations of the early part of the period were made 

 possible by the land connections brought about by deformative 

 movements. The extent of the connection of North America with 

 Asia at the northwest and with Europe at the northeast respectively, 

 is uncertain, but there is conclusive evidence that there were good 

 migratory routes for land mammals in both directions during a part 

 of the period. There are also strong hints that the connection 

 afforded passage for some species, but not for others, due perhaps to 

 the increasing cold toward the end of the period. This low tempera- 

 ture, with its effect on intermigration, was perhaps the chief factor 

 in developing the difference between the mammals of the Old World 

 and the New. 



The connection between North and South America introduced 

 a biological movement of much interest. There appears to have 

 been no effective isthmian thoroughfare for land animals between 

 the earliest Eocene and the Pliocene. During the Eocene con- 

 nection, a few North American mammals seem to have sent repre- 

 sentatives into South America, and these had evolved there on 

 distinctive lines in the interval. A remarkable group of sloths, 

 armadillos, and ant-eaters had developed from an edentate stem; 

 strange hoofed animals of orders unknown elsewhere had arisen from 

 some very primitive ungulate form; and the monkeys of the South 

 American type had evolved probably from a North American Eocene 

 lemuroid. That the connection of the continents in the Eocene 

 was only partial or temporary seems to be implied by the absence 

 in South America of most of the great North American groups. 

 The absence of proboscidians in South America implies lack of con- 

 nection between that continent and Africa, where these forms de- 

 veloped during the Eocene and Miocene; but the many marsupials 

 of South America, similar to those of Australia, imply either land 

 connection between those continents, or striking parallel evolution. 

 The South American mammalian fauna at the beginning of the 

 Pliocene is a striking instance of evolution on a large scale in com- 

 parative isolation, and in relative freedom from the severe stimulus 

 of effective competition, powerful carnivores, and shifting geo- 

 graphic relations. 1 



1 Reports of the Princeton University expedition to Patagonia, 1896-99 



