J. P. Smith — Periodic Migrations. 217 



Art. XX. — Periodic Migrations oetween the Asiatic and 

 the American Coasts of the Pacific Ocean; by James 

 Pekkin Smith. 



The fossil marine faunas of the western coast of the United 

 States, of British Columbia, Alaska, Japan, and India are 

 fairly well known, especially in the Mesozoic era. These 

 faunas are now similar, now different in the various provinces, 

 or in parts of them, presenting an apparently inextricable con- 

 fusion not capable of any rational explanation. But this con- 

 fusion is only apparent, for when studied in succession, a 

 regular scheme can be traced in the relationships and diversities 

 of this ancient region. And the changes that are noted point 

 to changes in physical geography that would be insignificant 

 in themselves, compared with continental uplifts and subsi- 

 dences, though by no means insignificant in their effects. 

 Also there is abundant independent physical evidence that 

 these changes really took place, so they are in no sense merely 

 hypothetical. 



Briefly stated, the facts are these. There is at present a 

 remarkable similarity in the living marine molluscs of the 

 western coast of North America and of the eastern coast of 

 Asia in approximately the same latitudes, and this similarity 

 can be traced back with certainty until the Lower Trias, and 

 probably even below that. But the resemblance is not con- 

 tinuous, there being periods in which the faunas were unlike, 

 and these periods of interruption recur several times, although 

 not regularly. It is clear that no migration is taking place 

 between the opposite sides of the Pacific now, and equally 

 clear that such migration did go on in comparatively recent 

 geologic time, when a large part of the present species of 

 marine invertebrates already existed. It remains to state the 

 facts in succession, and then to show how the intermigration 

 must have taken place, and the cause of the periodic interrup- 

 tions, reasoning back always from modern conditions to those 

 of the past. 



Ancient Faunal Relations. 



Paleozoic time. — We know little with certainty of the 

 faunal geography of the Paleozoic of the western coast of 

 America except that during the Carboniferous the con- 

 nection seems rather to have been with northern Asia than 

 with the interior of North America. The kinship of American 

 and Asiatic forms can not be charged to universality of faunas 

 or physical conditions, for we know that there was nearly as 

 much provincial differentiation in the marine Carboniferous as 

 there is now. 



Am. Jour. Sci.— Fourth Series, Yol. XVII, No. 99.— March, 1904. 

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