686 JAMES PERRIN SMITH 



sounds plausible, viewed in the light of what we know of the 

 distribution of Spirilla. But the living Pearly Nautilus is not 

 distributed by currents away from the region of its present 

 habitat, and in studying fossil faunas we find that the cephalo- 

 pods had little wider distribution than brachiopods and pelecy- 

 pods, animals usually fixed in station during most of their life, 

 and able to migrate only during the larval stage. Another 

 argument against the current-distribution hypothesis has been 

 brought up by Dr. A. Tornquist, that the fossil ammonites of 

 Jurassic and Cretaceous age are distributed approximately 

 according to climatic zones. 



The geologic record has been kept by the inhabitants of sub- 

 merged continental or island shelves, and their dispersion can- 

 not have been accidental or individual, but was faunal. The 

 means and the reason for this migration are furnished by changes 

 in physical geography. Any rising or sinking of shore lines 

 would drive the inhabitants from their dwelling places ; any 

 newly opened connections between regions that before were sep- 

 arated would cause an intermingling of different faunas. An 

 example of this is going on before our eyes today ; the Red Sea 

 and the Mediterranean have faunas as distinct as if they occurred 

 on opposite sides of the world, but since the opening of the Suez 

 Canal, intermigration has already begun, and in this present age 

 will be recorded an inter-regional invasion comparable with 

 those that took place in remote geologic time. And no doubt 

 to some future geologist this record will be just as clear as those 

 we have of similar changes in the past. Each great change in 

 the outlines of continents must also have caused great changes 

 in the direction of marine currents. Thus in the great subsi- 

 dence of land in northern Eurasia that caused the transgression 

 of the Upper Jurrasic sea must have opened the way for the cold 

 current that came from the northwest along the Pacific shore of 

 North America, bringing a Boreal fauna into temperate latitudes. 

 Something similar to this would happen if, at some future time, 

 the old dismembered Antillean continent were raised to its for- 

 mer position ; the Gulf Stream could not enter the warm waters 



