358 Darwin, and after Darwin. 



of a bird the third and fourth digits are completely 

 united by skin ; " so that in feather-footed pigeons, 

 not only does the exterior surface support a row of 

 long feathers, like wing-feathers [which, as just stated, 

 may in some cases be obviously differentiated into 

 primaries, secondaries and tertiaries], but the very 

 same digits which in the wing are completely united 

 by skin become partially united by skin in the feet ; 

 and thus by the law of correlated variation of homo- 

 logous parts, we can understand the curious connexion 

 of feathered legs and membrane between the two outer 

 toes V The illustration is drawn from the specimen 

 to which I have referred. 



Many similar instances of the same law are to be 

 met with throughout organic nature ; and it is evident 

 that in this principle we find a conceivable explanation 

 of the origin of such adaptive structures as could not 

 have been originated by natural selection acting directly 

 upon themselves : they may have been originated by 

 natural selection developing other adaptive structures 

 elsewhere in the organism, the gradual evolution of 

 which has entailed the production of these by correla- 

 tion of growth. And, if so, when once started in this 

 way, these structures, because thus accidentally useful, 

 will now themselves come under the direct action of 

 natural selection, and so have their further evolution 

 determined with or without the correlated association 

 which first led to their inception. 



Of course it must be understood that in thus apply- 

 ing the principle of correlated growth, to explain the 

 origin of adaptive structures where it is impossible to 

 explain such origin by natural selection having from 

 1 Variation of Plants and Animals, vol. ii. p. 315. 



