340 'DESCENT OF MAN '—EXPRESSION. [1872. 



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 Do- 

 mestication ') 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 function- 

 ally important ; 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 re- 

 marked I have not attended much of late years to such ques- 

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

 work at easier subjects. 



Believe me, yours very sincerely, 



Ch. Darwin. 



I hope and trust that you will throw light on these points. 



P.S. — I will add another remark which I remember oc- 

 curred to me when I first read M. Wagner. When a species 



