318 OKIGIN OF LIFE IN AMEEICA 



pressing my belief, contrary to the opinion of almost all those 

 who had occupied themselves with this problem, that these 

 earlier Asiatic immigrants took an entirely different route 

 from the later ones. The opinion I had formed was founded 

 on a variety of circumstances. 



If the theory I had formulated is correct, namely that the 

 effects presented by the Glacial Epoch were due to the 

 simultaneous casing of the Arctic Ocean from the Atlantic 

 and Pacific by the formation of two land bridges, one 

 of which connected North America with Europe in the 

 east, while the other joined North America and Asia in 

 the west (see Fig. 7), the warmer climate in pre-Glacial 

 times must have been due to the fact that the Arctic 

 Ocean then received a greater amount of heated water 

 than it does now. That the two land bridges must have existed 

 at nearly the same time seems to be amply demonstrated from 

 biological evidence. During the Miocene Period the climate 

 in the extreme north of Europe and North America must have 

 been much milder than in Pliocene times. The evidence from 

 northern Europe is against the supposition that the Gulf 

 Stream entered the Arctic Ocean more freely during the 

 Miocene Period than it does at present. The Japanese 

 "Kuroshiwo," or possibly some even more powerful marine 

 current, must have passed, therefore, entirely into the basin 

 of the Arctic Ocean across Bering Strait. I remarked before 

 (p. 96) that the appearance of certain Pacific species of 

 mollusks in the English Crag deposits may perhaps be due 

 to this cause. It was not until Pliocene times, according to 

 Professor J. P. Smith,* that the marine faunas of Japan 

 and the western coast of North America began to be re- 

 markably similar, thus implying that a migration at that time 

 took place along a continuous shore-line. The Californian 

 Miocene marine deposits seem mostly to contain endemic 

 species with a slight admixture of southern and circumboreal 

 ones. All this evidence favours the view of an open Bering 

 Strait in Miocene times, and a closed one during the Pliocene 

 Period. Dr. Dall f states that the Kenai leaf beds in Alaska 

 (now generally considered of Eocene or Oligocene age) are 



* Smith, J. P., " Periodic Migrations," pp. 225226. 

 t Dall, W. H., ''Correlation Papers Neocene," p. 251. 



