'DESCENT OF MAN' EXPRESSION. [1872. 



knowledge I did not make nearly sufficient use of the views 

 which you advocate ; and I almost wish I could believe in its 

 importance 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 

 continued selection of the fleetest individuals, without any 

 separation. 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 Aits- 

 land, 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 

 species 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 

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

 the least satisfactory to me unless it clearly explains such 



