1872.] ISOLATION. 159 



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 

 process of unconscious selection. ... I 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. 

 Nevertheless I cannot doubt that many new species have 

 been simultaneously developed within the same large conti- 

 nental area ; and in my ' Origin of Species ' I endeavoured 

 to explain 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 over- 

 looked 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. 

 Modifications thus caused, which are neither of advantage nor 

 disadvantage to the modified organism, would be especially 

 favoured, as I can now see chiefly 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 bearino- 



