228 J. P. Smith — Periodic Migrations. 



land animals and plants between Asia and America. And 

 indeed this may be so, for a subsidence that would cut off 

 effectually all land organisms might not interfere with the 

 migration of marine animals living at moderate depths. For 

 instance, Haliotis, a distinctly Asiatic type, is not known on 

 the west coast of America before the upper San Pedro epoch, 

 and has now made its way southward below California. On 

 the other hand, we must remember the conservative character 

 of marine faunas. Where in the land Pleistocene faunas most 

 of the species have been replaced by others, of the marine 

 Pleistocene animals only a very small percentage has become 

 extinct. We must also note that, of the species now common 

 to the two sides, a large proportion is known to have existed 

 in Tertiary time, and probably nearly all in Pleistocene. Tn 

 the time that has elapsed since the two provinces w^ere 

 separated, Lucina acutilineata, which abounded during the 

 Pliocene in both Japan and California and is still living in 

 Puget Sound, has become extinct in Japan. Mya arenaria, 

 also abundant in the Pliocene in both provinces, has become 

 extinct in California, while it still persists in Japan. It has, 

 however been introduced artificially late in the nineteenth 

 century and now abounds in most of the bays on the west 

 coast. On the Alaskan coast Mya arenaria did not become 

 extinct, though it did not make its way down io California. 

 It is also probable that Pecten caurinus on the west coast and 

 P. jessoensis in Japan have become differentiated from their 

 common ancestors. 



All this points to a rather ancient separation, not later than 

 the upper San Pedro epoch, at which time the warm- water 

 fauna came up from the south, and never reached Japan. 



Summary. 



Present physiography. — The living faunas of the Japanese 

 province and of the western coast of North America are rather 

 closely allied, with a large number of species in common, and 

 they live under approximately the same conditions, although 

 they are in widely separated regions. Between them lies a 

 stretch of shore-line running up to lat. 60° N., around the 

 southern shores of Alaska and the Aleutian Islands, but inter- 

 rupted by the deep channel east of Kamchatka. Also there is 

 a great difference of temperature between them. The warm 

 Japan current, with an average maximum temperature of 86° 

 F., flows past Japan, swings to the northeast, south of the 

 Aleutian chain towards America, and parts off Puget Sound, 

 one branch flowing northwestward along the Alaskan coast, 

 and the main branch southeastward down towards California. 



