Mr. Darwin, who has paid more attention to this matter than any other writer, has shown, in considerable detail, that all the parts of any given organism are so intimately bound together, or so mutually dependent upon each other, that when one part is caused to change by means of natural selection, some other parts are very likely to undergo modification as a consequence. For example, there are several kinds of domesticated pigeons and fowls, which grow peculiar wing-like feathers on the feet. These are quite unlike all the other feathers in the animal, except those of the wing, to which they bear a very remarkable resemblance. Mr. Darwin records the case of a bantam where these wing-like feathers were nine inches in length, and I have myself seen a pigeon where they reproduced upon the feet a close imitation of the different kinds of feathers which occupy homologous positions in the wing--primaries, secondaries, and tertiaries all being distinctly repeated in their proper anatomical relations. Furthermore, in this case, as in most cases where such wing-feathers occur upon the feet, the third and fourth toes were partly united by skin; and, as is well known, in the wing 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 homologous parts, we can understand the curious connexion of feathered legs and membrane between the two outer toes[46]." The illustration is drawn from the specimen to which I have referred. [46] _Variation of Plants and Animals_, vol. ii. p. 315. [Illustration: FIG. 117.--Feather-footed pigeon. Drawn from nature.] 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 correlation 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.