226 J. P. Smith — Periodic Migrations. 



commonly regarded as a chain of new volcanoes, granite 

 appears in several places.* 



At this time the marine faunas of Japan and the western 

 coast of America begin to be remarkably similar, with many 

 species identical, which can only mean that intermigration had 

 set up along the shore line. And the identical species are not 

 merely circumboreal, for many of them have never been found 

 in the Boreal region. A list of these, and their geologic range, 

 will be found on the table on page 229. 



The rise of the land in the northern hemisphere, as shown 

 by the distribution of land and fresh-water animals, would cut 

 off the southward cold current from the Arctic Sea, and pre- 

 vent the chilling of the Japan current south of Bering Sea. 

 The Japan current would then warm the shores of Alaska, 

 and produce a mild temperature along the old shore line from 

 Japan to California. That it did so is shown by the similarity 

 of the Pliocene faunas of the two now separated regions. At 

 first there was naturally a mixture of Boreal and Japanese 

 forms, for 1). BraUnsf has shown that the Pliocene of Japan 

 is related to the "Crag" of England; and the upper Pliocene 

 of California appears to indicate a temperature of the sea-water 

 somewhat lower than at present. At any rate, it is clear that 

 the climate around the North Pacific in Pliocene time was 

 merely temperate; this is shown by the fact that while we 

 have an immigration of Japanese species, no Indian forms 

 came with them, as they did in the Upper Cretaceous, when 

 the climate appears to have been subtropical. 



Pleistocene and Recent Faunal Relations. 



San Pedro epoch. — With the beginning of the Pleistocene 

 the same conditions existed as in the upper Pliocene. Japanese 

 species still abound in the marine fauna, and the character is 

 still somewhat boreal,J which may be due to a survival of 

 forms that came in during the colder Pliocene epoch. The 

 land connection with Asia still existed, and free exchange of 

 land animals between Asia and America still went on. 



As the waters of the Californian coast gradually became 

 warmer, Mexican species began to creep northward, and in the 

 upper San Pedro fauna we find a number of species that now 

 live only in the Tropics, and have become extinct on the Cali- 

 fornian coast.§ This does not mean that connection with Japan 



* J. E. Spurr, U. S. Geol. Survey, 20th Ann. Eept., Part vii, 234, 1900. 



f Geology of the Environs of Tokio, Mem. Science Dept., Univ. of Tokio, 

 No. 4, pp. 1-82, 1881. 



% Delos and Ralph Arnold, The Marine Pliocene and Pleistocene Strati- 

 graphy of the coast of Southern California, Jour. Geol., vol. x, No. 2, pp. 

 117-138 (1902). 



§ Ralph Arnold, Mem. Calif. Acad. Science, iii, 29 et seq., 1903. 



