318 
ORIGIN OF LIFE IN AMERICA 
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 closing 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. Dali f states that the Kenai leaf beds in Alaska 
(now generally considered of Eocene or Oligocene age) are 
* Smith, J. P., “ Periodic Migrations,” pp. 225—226. 
t Dali, W. H., “Correlation Papers—Neocene,” p. 251. 
