io6 Darivin, and after Dai^zvin, 



note merely because I believed that you had come to a similar 

 conclusion, and I like much to be in accord Avith you. I believe 

 I was mainly deceived by single variations offering such simple 

 illustrations, as when man selects [i.e. isolates]^. 



Again, somewhere about the same time, he wrote 

 to Moritz Wagner : — 



Although I saw the effects of isolation in the case of islands 

 and mountain-ranges, and knew of a few instances of rivers, 

 yet the greater number of your facts were quite unknown to me. 

 I now see that, from the want of knowledge, I did not make 

 nearly sufficient use of the views which you advocated 



Now it would be easy to show the justice of these 

 self-criticisms by quoting longer passages from earlier 

 editions of the Origin of Species ; but as this, in view 

 of the above passages, is unnecessary, we may next 

 pass on to another point. 



The greatest oversight that Wagner made in his 

 otherwise valuable essays on geographical isolation, 

 was in not perceiving that geographical isolation is only 

 one among a number of other forms of isolation : 

 and, therefore, that although it is perfectly true, as 

 he insisted, that polytypic evolution cannot be effected 

 by natural selection alone, it is very far from true, 

 as he further insisted, that geographical isolation is 

 the only means whereby natural selection can be 

 assisted in this matter. Hence it is that, when 

 Darwin said he had not himself " made nearly 

 sufficient use" of geographical isolation as a factor 

 of specific divergence, he quite reasonably added that 

 he could not go so far as Wagner did in regarding 

 such isolation as a condition, sine qua nan, to diver- 

 gent evolution in all cases. Nevertheless, he adds 



^ Life and Letters, vol. iii. pp. 157-8. * Ibid. pp. 157-8. 



