146 THE ORIGIN OF SPECIES 



exceptional cases of the same species, now living at dis 

 tant 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 ex- 

 istence of the same species on the summits of distant 

 mountain ranges, and at distant points in the arctic and 

 antarctic regions; and secondly (in the following chap- 

 ter), the wide distribution of fresh- water 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 oc- 

 casional means of transport, the belief that a single birth- 

 place 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 re- 

 gion 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 modification. A volcanic island, for instance, up- 



