I052 PRINCIPLES' OF STRATIGRAPHY 



land, like the continents of North and South America, are perhaps 

 the most important. For since they form a continuous barrier 

 across the warmer portions of the ocean, extending into the cold 

 regions, the migration of the warmer- water species from one side 

 to the other is prevented. Thus the marine faunas on the opposite 

 sides of North or South America differ widely. 



By the application of this- principle to the Cambric faunas of 

 North America and Europe, it has become apparent that a more 

 or less continuous land mass, sufficient to prevent intermingling 

 of faunas, separated the Atlantic and Pacific oceans in Cambric 

 time. This land barrier must have extended from New England 

 northeastward, joining this land-mass to the Scottish highlands, 

 and southward, forming the Appalachian old-land, which was 

 joined with the continent of South America. On opposite sides of 

 this land-mass different faunas flourished ; in the lower Cambric 

 the Olenellus fauna on the west and the Holmia fauna in the At- 

 lantic; in Middle Cambric time the Olenoides and Paradoxides 

 faunas flourished in these respective basins (see the maps in 

 Grabau-17). (See also Figs. 264a-c.) 



The only topographic element of importance in lakes and other 

 bodies of standing fresh water is the enclosing land mass, which 

 prevents direct migration from one lake into another. In rivers 

 the presence of rapids and falls may become a barrier to all but the 

 most agile of swimmers. Here, however, the barrier is most ef- 

 fective against upstream migration. The banks likewise constitute 

 barriers. 



Terrestrial animals and plants are often prevented from migrat- 

 ing or becoming dispersed by the existence of topographic barriers 

 in the form of mountains of great height and consequent range in 

 climate ; by large bodies of water, deep river gorges and impass- 

 able streams : and by extensive desert tracts ; the last being produced 

 by a combination of the topographic and climatic factors. For ani- 

 mals of a more limited range of habitat, minor topographical bar- 

 riers become restrictive, as shown by the distribution of the Hawai- 

 ian tree snails {Achatincllidcv) , already referred to, for which the 

 exposed ridges separating adjoining ravines form almost impass- 

 able barriers. 



The Organic Factors. 



The organic conditions of the three realms likewise exert an 

 important influence on the distribution of animal and plant life. 



