J. P. Smith — Periodic Migrations. 225 



species of this fauna may be identical with Atlantic forms, at 

 any rate some are closely related, and it is probable that the 

 passage lay to the south of California. 



Miocene. — In the middle Tertiary the passage to the Atlan- 

 tic seems to have been closed, and there is no evidence that 

 communication was resumed with Asia. The Miocene fauna 

 of California seems to have been largely endemic, for no 

 Atlantic species are found in it, and the only possible admix- 

 ture consists of forms from the south, and of circumboreal 

 species that made their way down from the north. But 

 towards the end of Miocene time the land appears to have 

 risen in the north, cutting off the Arctic Ocean from the 

 Pacific, and allowing land plants to migrate from Asia to 

 North America. Asa Gray* has shown that in the Miocene 

 northeastern Asia and northwestern America were connected, 

 that over those regions there existed a flora like that of warm 

 temperate latitudes at the present time, and that this connec- 

 tion persisted almost to the beginning of the Glacial epoch. 



We have no evidence that a migration of marine inverte- 

 brates from Asia began as early as the upper Miocene, but they 

 would naturally be slower in their movements than land plants, 

 and consequently would lag behind them. The Tertiary 

 uplift of land in the northern hemisphere may be correlated 

 with the widespread orogenic uplift of the Coast Ranges on 

 the Pacific side of North America, which in California and 

 Oregon is known to have come at the end of the Miocene, 

 and before the Pliocene beds were laid down. 



Pliocene. — There is good geologic evidence that the land- 

 bridge between Asia and America still existed in the Pliocene, 

 for there seems to have been a constant interchange of verte- 

 brates in that quarter, f in the Miocene, Pliocene, and early 

 Pleistocene. Also students of other groups find it necessary 

 to postulate such a connection to explain the migration of 

 animals. A. E. OrtmannJ says that the identity of some of 

 the fresh-water crustaceans in Siberia and Alaska proves a 

 recent connection of those parts, and that this union of the 

 continents began in the middle of the Cretaceous and lasted 

 into the lower Pleistocene. G. M. Dawsong is of the opinion 

 that in the Pliocene the Pacific coast of North America stood 

 about 900 feet higher than now, which would be ample to 

 connect the two continents, and cut oh* the cold water from the 

 North Pacific. And we still have evidence of this former 

 elevation of the Alaskan region, for in the Aleutian Islands, 



*Amer. Jour. Sci. (3), cxvi, 195, 1878. 



f H. F. Osborn, Science, xi, 571, 1900. 



% Proc. Amer. Phil. Soc, xli, 291, 299 and 316, 1902. 



§ Quoted in Dana's Manual of Geology, 949, 1895. 



