112 ORGANIC EVOLUTION 



are present between these two areas to prevent free pas- 

 sage from one to the other. There is also a correlation 



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between the kinds of barriers present and the kinds of 

 animals and plants held in check by them. Aquatic ani- 

 mals and plants are restricted by the intervention of land 

 areas. Terrestrial organisms are held back by the presence 

 of large bodies of water. Animals and plants adapted to 

 warm climates may be unable to cross high mountain ranges 

 whose summits will have a cold climate. Dry regions will 

 check organisms which are adapted to life in fertile areas. 

 Desert species will not readily pass a forest barrier or a 

 region of marshes. 



Observe the conditions on some of the islands off the 

 west coast of South America. Their faunas and floras, 

 while different from those of the mainland because of their 

 isolation and different environment, are still quite closely 

 related to those of the mainland, presenting just the con- 

 ditions we would expect on the supposition that they are 

 descended from forms which migrated from the mainland at 

 some remote period, migration having since been suspended. 

 Similarly we explain the resemblance between the fauna 

 and flora of the west coast of North America and those of 

 eastern Asia by the fact that at one time, when the climate 

 of Alaska was mild, migration across Behring Straits was 

 possible, and by our belief that the Asiatic forms once estab- 

 lished in this country and American forms once having 

 crossed into Asia, communication having then been broken 

 off, the forms thus separated would diverge by evolution. 



The flora of the higher altitudes in the White Mountains 



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of New Hampshire shows a remarkable resemblance to that 

 of Labrador. This suggests that the White Mountain flora 



