316 ON THE ORIGIN OF SPECIES 



enormously remote geological period, so certain species 

 have migrated over vast spaces, and have not become 

 greatly modified. 



On these views, it is obvious, that the several species 

 of the same genus, though inhabiting the most distant 

 quarters of the world, must originally have proceeded 

 from the same source, as they have descended from the 

 same progenitor. In the case of those species, which 

 have undergone during whole geological periods but 

 little modification, there is not much difficulty in believ- 

 ing that they may have migrated from the same region ; 

 for during the vast geographical and climatal changes 

 which will have supervened since ancient times, almost 

 any amount of migration is possible. But in many other 

 cases, in which we have reason to believe that the species 

 of a genus have been produced within comparatively 

 recent times, there is great difficulty on this head. It 

 is also obvious that the individuals of the same species, 

 though now inhabiting distant and isolated regions, must 

 have proceeded from one spot, where their parents were 

 first produced : for, as explained in the last chapter, it 

 is incredible that individuals identically the same should 

 ever have been produced through natural selection from 

 parents specifically distinct. 



We are thus brought to the question which has been 

 largely discussed by naturalists, namely, whether species 

 have been created at one or more points of the earth's 

 surface. Undoubtedly there are very many cases of 

 extreme difficulty, in understanding how the same 

 species could possibly have migrated from some one 

 point to the several distant and isolated points, where 

 now found. Nevertheless the simplicity of the view that 

 each species was first produced within a single region 

 captivates the mind. He who rejects it, rejects the 

 vera causa of ordinary generation with subsequent 

 migration, and calls in the agency of a miracle. It is 

 universally admitted, that in most cases the area in- 

 habited by a species is continuous ; and when a plant 

 or animal inhabits two points so distant from each 

 other, or with an interval of such a nature, that the 



