96 ORIGIN OF LIFE IN AMERICA 



Professor Arnold. At any rate, it is reasonable to infer that 

 a rise to a higher temperature of the northern Pacific Ocean, 

 coupled with an increased conveyance of Asiatic species to 

 the American coasts and a northward advance of southern 

 forms must have coincided with the closing of Bering 'Strait. 

 And it was not until Pliocene times, according to Professor 

 J. iP. Smith,* thait the marine faunas of Japan and the western 

 coast of America began to be remarkably similar, many species 

 being identical. From this fact we must conclude that inter- 

 migration between 'the two continents had set along a northern 

 shore line. During the preceding 'Miocene Period the marine 

 fauna of California consisted of endemic species mixed with' 

 southern and circumboreal ones, but without any Asiatic 

 admixture. Consequently there was probably a wide com- 

 munication between the Pacific and the Arctic Ocean, favour- 

 ing the entrance into the latter of a warm current which 

 profoundly affected the Arctic Regions. The curious relation- 

 ship to Pacific moUusks which is noticeable among some 

 forms of the English Crag deposits may possibly date from 

 this theoretical Miocene current, which may have carried 

 marine species right across the Polar Seas to Europe. 



At the beginning of the Pleistocene Period, the same con- 

 ditions existed, according to Professor Smith, as in the Upper 

 Pliocene. As the waters of the Californian coast gradually 

 became warmer, he remarks, Mexican species began to creep 

 northward. But this, he says, does not mean that connection 

 with Japan was cut off. The continuation of the conditions 

 that permitted Japanese species to migrate to California, 

 merely allowed marine animals to make their way up the 

 American coast also. Here I must beg to differ from Pro- 

 fessor Smith. If a change in the fauna of the upper Pleis- 

 tocene of California took place as asserted, that change was 

 in all probability due to a gradual sinking of the land in 

 the north, for a moderate subsidence in northern Alaska at 

 any rate has been recorded by Dr. Dall f during later Pleisto- 

 cene times. A gradual modification was thus brought about 

 in the disposition of land and water, the continents of Asia 

 and North America slowly assuming their present shapes. 



* Smith, J. P., "Periodic Migrations," pp. 225—226. 

 t Dall, W. H., " Neocene of North America," p. 278. 



