230 J. P. Smith — Periodic Migrations. 



to about 66° F, It tempers the Alaskan waters, but makes the 

 waters along the shores of California colder than they should 

 be, as compared with similar latitudes elsewhere. These cur- 

 rents have been fully described by Professor George David- 

 son,* who has made many hydrographic investigations in that 

 region ; and by Dr. W. H. Dall,f who verified much of the 

 work of Professor Davidson, but showed that there was no 

 northward branch of the Japan current extending up into 

 Bering Sea. 



At present the migration of shallow water species is stopped 

 by the depth of the channel at the end of the Aleutian chain, 

 and also by the cold water that extends south westward from 

 Bering Sea. But a rise of 200 meters would close Bering 

 Strait, and about one-half of Bering Sea, giving a shore-line 

 coinciding approximately with a great circle. It would then 

 leave the Aleutian chain as a long narrow peninsula reaching 

 out from Alaska towards Siberia, separated from Kamchatka 

 by a rather narrow but deep channel ; while the mainland of 

 Alaska and Siberia would be united by a broad land-bridge. 

 This change in the height of the land would cut off all influx 

 of cold water from the Arctic Sea, and the Japan current, not 

 being chille^ by cold water from Bering Sea, would still be 

 warm along the Aleutian Islands and the Alaskan coast, and 

 no doubt the tempering effect would be felt even as far south 

 as California. A rise of 2,000 meters would not connect the 

 Aleutian chain with Kamchatka, but at least would give a 

 stretch of shoal water along which migration would be easy 

 for shore forms. In any case there would be a shore line with 

 temperate or warm water all the way from Japan to California. 



While it is not likely that the land in the northern part of 

 the Pacific has, in recent geologic times, stood 2,000 meters 

 higher than now, it has certainly stood several hundred feet 

 higher, and whether much or little, we know from the migra- 

 tions of land plants and animals between Asia and America 

 that there has been a land-bridge. Now in any case, whether 

 in the present or in the past, similar contemporaneous faunas 

 mean similar conditions, and identical species mean immigra- 

 tion from one region to the other, or from a third region to 

 both. In the case of Japan and the west coast of America the 

 only outside region that can have furnished elements to both 

 is the Boreal region, and circumboreal species in both prov- 

 inces are well known. But a large majority of the species 

 now common to both provinces are not circumboreal. Thus 

 intermigration is the only satisfactory explanation of the pre- 



* Report Supt. U. S. Geodetic Survey, 1867, Appendix No. 18, p. 202,1869. 

 f Rept. Supt. U. S. Coast and Geod. Survey for 1880, Appendix No. 16, p. 

 322. 



