J. P. Smith — Periodic Migrations. 227 



was cut off, but that the continuation of the conditions that 

 allowed Japanese species to migrate to California finally allowed 

 marine animals to make their way up the coast also. 



Glacialists postulate an elevation of the land in the northern 

 hemisphere, in the period preceding and during the Glacial 

 epoch ; this undoubtedly cut off the migration of land animals 

 and plants between Asia and America. When this affected 

 intercommunication along the shore-line we do not know, but 

 after the close of the San Pedro epoch the Asiatic immigration 

 ceased, and also the subtropical elements of the marine fauna 

 of California became extinct. With the subsidence following the 

 Glacial epoch conditions returned to the normal, and intermi- 

 gration with Japan was not resumed. It was too cold for the 

 perpetuation of the warm-water species from the south, though 

 not too cold for the Japanese species to live on in the Califor- 

 nian waters. But the Asiatic colonists in America were not 

 replenished by immigration from the mother country, and 

 those that are found in the Californian province are merely 

 survivors. 



Relations of the living faunas of the west coast to that of 

 Japan. — The table on page 229 shows the living species that 

 are common to the western coast of North America and the 

 Japanese province. The number of species is very large, 

 especially when we consider the fact that they are in different 

 zoologic regions, and separated by more than five thousand 

 miles. Migration of shore forms can not possibly be going on 

 now, for while the distance is no bar to them, the deep water 

 at the end of the Aleutian chain of islands would effectually 

 check all passage in either direction. And the sudden changes 

 in temperature through which marine animals would have to 

 pass are an equally effectual barrier. We have seen already 

 how even a circumboreal species, as Purpura lapillus, is 

 checked in its southward passage where the cold current from 

 Bering Sea meets the warm Japan current. Such changes are 

 even more impassable to warm-water species. 



In the present similarity of the marine faunas of Japan and 

 the western coast of America we have an example of provinces 

 that were recently connected, but which are now separated by 

 deep water and by differences of temperature in between, 

 while the conditions still remain similar in the two provinces. 

 This separation, however, has- existed long enough for some of 

 the species to have become differentiated by evolution, for 

 some to have become extinct in one province while still living 

 in the other, and for the total faunas to have become much 

 changed by immigration from other regions. 



At first sight it would seem that the intermigration of the 

 marine faunas has been more recent than the exchange of the 



