i872.] ISOLATION. 337 



can assure you, the highest gratification of which I am capa- 

 ble. . . . 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 un- 

 known to me. I now see that from the want of knowledge I 

 did not make nearly sufficient use of the views which you 

 advocate ; and I almost wish I could believe in its impor- 

 tance to the same extent with you ; for you well show, in a 

 manner which never occurred to me, that it removes many 

 difficulties and objections. But I must still believe that in 

 many large areas all the individuals of the same species have 

 been slowly modified, in the same manner, for instance, as the 

 English race-horse has been improved, that is by the con- 

 tinued selection of the fleetest individuals, without any sepa- 

 ration. But I admit that by this process two or more new 

 species could hardly be found within the same limited area ; 

 some degree of separation, if not indispensable, would be 

 highly advantageous ; and here your facts and views will be 

 of great value. ... 



[The following letter bears on the same subject. It refers 

 to Professor M. Wagner's Essay, published in Das Auslandy 

 May 31, 1875 



C. Darwin to Moritz Wagner. 



Down, October 13, 1876. 



Dear Sir, — I have now finished reading your essays, 

 which have interested me in a very high degree, notwith- 

 standing that 1 differ much from you on various points. For 

 instance, several considerations make me doubt whether spe- 

 cies are much more variable at one period than at another, 

 except through the agency of changed conditions. I wish, 

 however, that I could believe in this doctrine, as it removes 

 many difficulties. But my strongest objection to your theory 

 is that it does not explain the manifold adaptations in struc- 

 ture in every organic being — for instance in a Picus for 

 climbing trees and catching insects — or in a Strix for catching 

 39 



