PRINCIPLES OF PALEONTOLOGIC CORRELATION 687 



of the Carribean, would be deflected, and the waters of the north- 

 western coast of Europe would be chilled. 



The horizons of America that represent periods of instability 

 of shore lines are the very ones that contain the remains of exotic 

 faunas, such as those of the Cuboides zone, the Intumescens zone, the 

 Chouteau limestone, the St. Louis beds, or the shifting zones of 

 the Coal Measures. These migrations must ail have been faunal 

 rather than individual, and can have been due only to physical 

 agencies acting slowly and on a large scale. No extraordinary 

 catastrophies need be appealed to as an explanation of this, for 

 similar phenomena are always going on before our eyes, in 

 the slow but ceaseless rising of some shores and sinking of 

 others. 



Land masses present an insuperable barrier to marine ani- 

 mals ; but if the bodies of land are short, and do not reach into 

 polar waters, animals can easily pass around the ends. Thus the 

 molluscan fauna of the Mediterranean does not differ greatly from 

 that of the English waters, because in the passage around the 

 peninsula of Spain, animals remain in temperate waters and under 

 nearly the same conditions. East and west land masses would, 

 therefore, not be effectual barriers, since they would not be 

 so likely to extend into frigid waters nor into very great differ- 

 ences of temperature. An example of this is the similarity of 

 marine faunas on the east and the west coast of Australia. 



On the other hand, the Isthmus of Panama separates two 

 faunas absolutely distinct from each other, although in the same 

 latitude, and under the same climatic conditions. Also the 

 Isthmus of Suez separates two totally distinct faunas, but these 

 belong to different regions and even to different climatic zones, 

 brought near together by the narrow strip of the Red Sea. A 

 similar case is known in the Jurassic formation, where the fauna 

 of western Europe stands sharply contrasted with that of Russia ; 

 even the characteristic genera are distinct, and this too in lati- 

 tudes not very different. These two types represent two seas of 

 different climatic zones, separated by a strip of land during the 

 later portion of Jurassic time. 



