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



animals at night, and so on ad infifutum. No theory is in 

 the least satisfactory to me unless it clearly explains such 

 adaptations. I think that you misunderstand my views on 

 isolation. I believe that all the individuals of a species can 

 be slowly modified within the same district, in nearly the 

 same manner as man effects by what I have called the pro- 

 cess of unconscious selection. ... 1 do not believe that one 

 species will give birth to two or more new species as long as 

 they are mingled together within the same district. Never- 

 theless I cannot doubt that many new species have been 

 simultaneously developed within the same large continental 

 area ; and in my * Origin of Species ' I endeavoured to ex- 

 plain how two new species might be developed, although 

 they met and intermingled on the borders of their range. It 

 would have been a strange fact if I had overlooked the 

 importance of isolation, seeing that it was such cases as that 

 of the Galapagos Archipelago, which chiefly led me to study 

 the origin of species. In my opinion the greatest error 

 which I have committed, has been not allowing sufficient 

 weight to the direct action of the environment, i.e. food, 

 climate, &c., independently of natural selection. Modifica- 

 tions thus caused, which are neither of advantage nor disad- 

 vantage to the modified organism, would be especially fa- 

 voured, as I can now see chiefiy through your observations, 

 by isolation in a small area, where only a few individuals 

 lived under nearly uniform conditions. 



When I wrote the ^ Origin,' and for some years afterwards, 

 I could find little good evidence of the direct action of the 

 environment ; now there is a large body of evidence, and your 

 case of the Saturnia is one of the most remarkable of which I 

 have heard. Although we differ so greatly, I hope that you 

 will permit me to express my respect for your long-continued 

 and successful labours in the good cause of natural science, 

 I remain, dear Sir, yours very faithfully, 



Charles Darwin, 



[The two following letters are also of interest as bearing 



