1872.] 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 Ausland, 

 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 I 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 



