138 SINGLE CENTRES OF CEEATION. [Chap. XII. 



hopelessly tedious to discuss all the exceptional cases 

 of the same species, now living at distant and separated 

 points, nor do I for a moment pretend that any 

 explanation could be offered of many instances. But, 

 after some preliminary remarks, I will discuss a few of 

 the most striking classes of facts ; namely, the existence 

 of the same species on the summits of distant mountain 

 ranges, arid at distant points in the arctic and antarctic 

 regions; and secondly (in the following chapter), the 

 wide distribution of freshwater productions ; and thirdly, 

 the occurrence of the same terrestrial species on islands 

 and on the nearest mainland, though separated by 

 hundreds of miles of open sea. If the existence of the 

 same species at distant and isolated points of the earth's 

 surface, can in many instances be explained on the view 

 of each species having migrated from a single birthplace ; 

 then, considering our ignorance with respect to former 

 climatal and geographical changes and to the various 

 occasional means of transport, the belief that a single 

 birthplace is the law, seems to me incomparably the 

 safest. 



In discussing this subject, we shall be enabled at the 

 same time to consider a point equally important for us, 

 namely, whether the several species of a genus which 

 must on our theory all be descended from a common 

 progenitor, can have migrated, undergoing modification 

 during their migration, from some one area. If, when 

 most of the species inhabiting one region are different 

 from those of another region, though closely allied to 

 them, it can be shown that migration from the one region 

 to the other has probably occurred at some former period, 

 our general view will be much strengthened ; for the 

 explanation is obvious on the principle of descent with 



