J. P. Smith— Periodic Migrations. 231 



sent distribution of most of the species that are common to 

 Japan and California. 



tinlike species living contemporaneously in similar condi- 

 tions can only mean separation by physical barriers ; these to 

 the marine shore-animals are : a land-mass ; deep water ; or 

 great difference of temperature in between. The two latter 

 are the only barriers that can ever have been interposed 

 between Japan and the west coast of America, and a rise of 

 200 meters would remove both. 



The hypothesis of former migration around the Alaskan- 

 Aleutian shore-line explains satisfactorily the close relationship 

 between the living marine animals of Japan and California, 

 while the present interruption and the length of time during 

 which it has persisted explain the unlikeness of the greater 

 part of the two faunas. A periodic recurrence of this inter- 

 ruption accounts for the periodically recurring unlikeness of 

 faunas of these two provinces in the past, and also for the fact 

 that the successive faunas of California do not form a genetic 

 series, but rather one showing periodically diverse origin and 

 characters. With this in mind we can find out where the 

 successive migrations came from, and why the present fauna 

 shows such a commingling of forms derived partly from Asia 

 and partly from the more southerly regions of America. 



Past physiography. — The old idea of uniformity of climate 

 all over the earth before the Tertiary period still remains as an 

 undercurrent in the minds of geologists, and stands in the way 

 of any theory that accepts the influence of differences of tem- 

 perature in causing faunal differences in the past. Stanton* 

 denies that the change of character of the Cretaceous fauna of 

 the west coast at the end of the Knoxville epoch, from the 

 Boreal to the Indian type, can be attributed to change in 

 climate. Others have criticized Neumayr'sf theory of climatic 

 zones in Jurassic and Cretaceous times, and there can be no 

 doubt that Neumayr carried his theories entirely too far, 

 reconstructing ancient physical geography on very little evi- 

 dence. But the fact that Neumayr was mistaken in many 

 things is no argument that the principle was wrong. JS T o 

 doubt geographic connections and presence or absence of 

 opportunities for free intercommunication were of just as 

 great importance as differences of temperature in governing 

 the distribution of faunas in the past, as well as now. But it 

 will not do to leave climate entirely out of the reckoning. 



The assumption of a uniform temperature over the earth 

 before the Tertiary period rests on an insecure basis. If any 



*Jour. Geol., v, 598, 1897. 



f Klimatische-Zonen wahrend der Jura- und Kreidezeit. Denkschr. k. 

 Akad. Wiss. Wien, xlvii, 1883. 



