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 
