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Next in importance, perhaps, to the elevations and 

 sinkings (traces of one or the other of which are more or 

 less manifest in almost every region of the world), 

 natural barriers may be cited, as presenting, not un- 

 frequently, insurmountable obstacles to the self- dissemi- 

 nation of the insect tribes. By natural barriers, how- 

 ever, I would be understood to imply natural primary 

 barriers, or, in other words, such as have continued as 

 barriers ever since the present animals and plants came 

 into existence upon the earth. For, the ocean (by way 

 of illustration) is a natural barrier ; and yet it is not 

 necessarily a primary one, as may be readily gathered 

 from the above remarks, in which the results of subsi- 

 dences are discussed, subsidences which have had the 

 effect of letting it in over portions of an already tenanted ', 

 and unbroken, continent. Mountain- chains, also, are 

 barriers ; but it may happen that they have not been so 

 from the beginning, as in instances, for example, where 

 they have been gradually upraised during periods geolo- 

 gically recent. But both sea and alpine ranges are 

 barriers, when (as usually happens) they have remained 

 as such since the creation of the several species which 

 now inhabit our globe. Mr. Darwin has acknowledged 

 this distinction, whilst commenting upon the marked 

 divergence of the faunas on the eastern and western 

 slopes of the Cordillera. " This fact," says he, ' ' is in 

 perfect accordance with the geological history of the 

 Andes; for these mountains have existed as a great 

 barrier since the present races of animals have appeared ; 



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