J. P. Smith — Periodic Migrations. 233 



fornian province are synchronous and similar, with many iden- 

 tical species, although they are in different geographic regions. 

 And there is no more reason to assume that the similar Meso- 

 zoic faunas of the two regions were not synchronous than 

 there is for the present time. The modern instance shows that 

 they may just as well have been synchronous as not. 



Biologists are often sceptical as to identity of species in 

 separated regions in the past, on the ground that the criteria 

 for determining fossils are not so exact as those applied to 

 recent forms. But this is also fallacious, since the recent 

 shells of Japan and California have been subjected to most 

 careful examination by critical conchologists, and many species 

 found to be identical. There is, therefore, no presumption 

 against the identity of similar species in the two regions of 

 Cretaceous, Jurassic, or even Triassic age. 



On the permanence of the shore-line. — The marine sediments 

 and their fossils around the North Pacific, from the Trias on 

 to the present time, show that the shore-line has been, during 

 all that time, approximately as it is now. There is no reason 

 to theorize about great changes in physical geography, when 

 such a simple matter as the periodic opening and closing of 

 Bering Strait by rising and sinking of the land in that quarter 

 will account satisfactorily for all the changes in character and 

 distribution of the marine faunas. E. Haug,* in his studies of 

 the distribution of Mesozoic formations and organisms, has 

 invented the theory that during Mesozoic time a great conti- 

 nent existed where the Pacific Ocean now is. Whatever may 

 be true of the rest of that ocean, there is no necessity for sup- 

 posing a continent to have existed formerly in the North 

 Pacific, when all the facts may be explained more easily on the 

 hypothesis of the permanence of the shore-line approximately 

 as it is now. This has nothing to do with the general propo- 

 sition of the permanence of continental plateaus and oceanic 

 basins, but merely proves a particular case, and suggests that 

 some of the ancient oceans, that are to be found on maps pur- 

 porting to represent the former continents and seas, may be 

 only epicontinental seas. 



* Les Geosynclinaux et les Airies continentales. Bull. Soc. Geol., France 

 (3), xxviii, 646. 



Stanford University, California. 



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

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