356 SINGLE CENTRES OF CREATION. 



area alone, and having subsequently migrated from that 

 area as far as its powers of migration and subsistence under 

 past find present conditions permitted, is the most probable. 

 Undoubtedly many cases occur in which we cannot explain 

 how the same species could have passed from one point to 

 the other. But the geographical and climatical changes 

 which have certainty occurred within recent geological 

 times, must have rendered discontinuous the formerly con- 

 tinuous range of many species. So that we are reduced to 

 consider whether the exceptions to continuity of range are 

 so numerous, and of so grave a nature, that we ought to give 

 up the belief, rendered probable by general considerations, 

 that each species has been produced within one area, and 

 has migrated thence as far as it could. It would be hope- 

 lessly 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, and at distant points 

 in the Arctic and Antarctic regions ; and secondly (in the 

 following chapter), the wide distribution of fresh-water pro- 

 ductions ; and thirdly, the occurrence of the same terrestrial 

 species on islands and on the nearest mainland, though sep- 

 arated 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 birth- 

 place, then, considering our ignorance with respect to former 

 climatical and geographical changes, and to the various 

 occasional 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 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 



