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” 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. 
