I 



Isolation. 27 



bodies of observable fact, which severally and collec- 

 tively go to verify, with an overwhelming mass of 

 evidence, the conclusion previously reached on grounds 

 of general reasoning. 



The facts of geographical distribution seem to me to justify 

 the following statements : — 



(1) A species exposed to different conditions in the different 

 parts of the area over which it is distributed, is not repre- 

 sented by divergent forms when free interbreeding exists 

 between the inhabitants of the different districts. In other 

 words, Diversity of Natural Selection without Separation does 

 not produce divergent evolution. 



(2) We find many cases in which areas, corresponding in 

 the character of the environment, but separated from each 

 other by important barriers, are the homes of divergent forms 

 of the same or allied species. 



(3) In cases where the separation has been long continued, 

 and the external conditions are the most diverse in points 

 that involve diversity of adaptation, there we find the most 

 decided divergences in the organic forms. That is, where 

 Separation and Divergent Selection have long acted, the 

 results are found to be the greatest. 



The 1st and 3rd of these propositions will probably be 

 disputed by few, if by any. The proof of the 2nd is 

 found wherever a set of closely allied organisms is so 

 distributed over a territory that each species and variety 

 occupies its own narrow district, within which it is shut by 

 barriers that restrain its distribution while each species of 

 the environing types is distributed over the whole territory. 

 The distribution of terrestrial molluscs on the Sandwich 

 Islands presents a great body of facts of this kind. 



