206 To the River Plate and Back 



North America into Asia and from Asia into North 

 America. While this was going on the two Americas 

 had no connection with each other at all. Subsequently, 

 however, a connection between North and South 

 America was established. This union of the two western 

 continents appears to have taken place long after a 

 connection between South America and the Antarctic 

 continent had ceased to exist. When North and South 

 America became united there took place an invasion 

 of South America by animals from North America 

 and a return wave of emigration from South America 

 into North America swept upward. From North 

 America there passed into South America the camel- 

 like animals, which had come into being on the plains 

 of what are now the regions of the Rocky Mountains. 

 The peccaries, the deer, the cats, the tapirs, emigrated 

 from North America, as did also the mastodon, the 

 latter animal representing an invasion from far-away 

 Asia by way of the Bering Sea land-bridge. From 

 South America there traveled northward the ground- 

 sloths, the opossums, the armadillos, the toxodonts, 

 and other creatures which had their origin upon South 

 American soil. These reciprocal movements probably 

 did not take place much before the close of the Pliocene, 

 and during the early Pleistocene. In caves in Pennsyl- 

 vania we have quite recently found the remains of a 

 number of animals, the nearest relatives of which occur 

 in the Pampean beds of Argentina. 



Certain of the friends of the writer have rather strenu- 

 ously advocated the view that the continent of South 

 America was at one time linked to the continent of 

 Africa by a land-bridge which reached across the 

 Atlantic Ocean from eastern and northern Brazil to 

 the nearest point of the African continent. This view 



