EIGHTH ANNUAL REPORT. 187 



nating in Africa, spread over the world, and even reached the 

 southern end of South America in Pliocene times. 



Other animals of this early period have probably survived, but 

 we are without such knowledge of the forest fauna of preglacial 

 and postglacial times as would enable us to trace them down to 

 existing forms. Our American deer, Odocoileus, the prong- 

 horn, peccaries and raccoons, probably are survivors of this same 

 fauna. But the fact remains that the great majority of the early 

 American mammals perished, probably because of their inability 

 to cross the Mexican deserts, and because of the destruction of 

 their normal food supply. 



EURASIATIC CONNECTION. 



During and after the glaciers came a new fauna, the imme- 

 diate ancestors of the predominating types of to-day. At inter- 

 vals throughout Tertiary times there apparently existed a broad 

 land connection with Eurasia, over the present site of Behring 

 Sea. This connection probably existed in the Lower Eocene, 

 Lower Oligocene, Middle Miocene, Upper Pliocene, and Lower 

 Pleistocene, and was interrupted in Middle and Upper Eocene, 

 Upper Oligocene and Lower Miocene times. 



We have several corresponding invasions from Eurasia. The 

 first of these invasions was in the Lower Eocene; the second in 

 the Lower Oligocene; the third, bringing in the Proboscidians, 

 in the Miocene ; the fourth occurred during the Lower and Middle 

 Pleistocene, bringing the ancestors of most of the typical Ameri- 

 can animals of to-day. 



This Eurasian land connection had a sufficiently temperate 

 climate during early Tertiary times to admit of the interchange 

 of animals which required either an unbroken forest or a tem- 

 perate climate, and consequently the faunae of the eastern and 

 western continents were very closely related, but as time went on 

 they became more distinct, until it is evident that some obstacle 

 existed, probably the increasing cold temperature of the ever- 

 narrowing land bridge. Animals which could sustain great cold 

 and long journeys still crossed, but the more southern types were 

 cut off. 



THE BOREAL THEORY. 



In this connection it is necessary to mention the recently ad- 

 vanced theory of the boreal origin of mammals, indeed of all 

 life, both animal and vegetable. The chief evidence, so far as it 



