1872.] ISOLATION. l6l 



through my mind of late years. In North America, in going 

 from north to south or from east to west, it is clear that the 

 changed conditions of life have modified the organisms in the 

 different regions, so that they now form distinct races or even 

 species. It is further clear that in isolated districts, however 

 small, the inhabitants almost always get slightly modified, and 

 how far this is due to the nature of the slightly different 

 conditions to which they are exposed, and how far to mere 

 interbreeding, in the manner explained by Weismann, I can 

 form no opinion. The same difficulty occurred to me (as 

 shown in my 'Variation of Animals and Plants under Domesti- 

 cation ') with respect to the aboriginal breeds of cattle, sheep, 

 &c, in the separated districts of Great Britain, and indeed 

 throughout Europe. As our knowledge advances, very slight 

 differences, considered by systematists as of no importance 

 in structure, are continually found to be functionally im- 

 portant ; and I have been especially struck with this fact in 

 the case of plants to which my observations have of late years 

 been confined. Therefore it seems to me rather rash to 

 consider the slight differences between representative species, 

 for instance those inhabiting the different islands of the same 

 archipelago, as of no functional importance, and as not in any 

 way due to natural selection. With respect to all adopted 

 structures, and these are innumerable, I cannot see how M. 

 Wagner's view throws any light, nor indeed do I see at all 

 more clearly than I did before, from the numerous cases 

 which he has brought forward, how and why it is that a long 

 isolated form should almost always become slightly modified. 

 I do not know whether you will care about hearing my 

 further opinion on the point in question, for as before 

 remarked I have not attended much of late years to such 

 questions, thinking it prudent, now that I am growing old, to 

 work at easier subjects. 



Believe me, yours very sincerely, 



Ch. Darwin, 

 vol. III. M 



