direction (produced by natural selection for the sake of better and
better adapting the structure to perform some particular function) ends
by beginning to adapt it to the performance of some other function. And,
whenever this happens to be the case, natural selection forthwith begins
to act upon the structure, so to speak, from a new point of departure.

So much, then, for the Duke's premiss--namely, that "every modification
of structure _must_ have been functionless _at first_, when it began to
appear." This premiss is clearly opposed to observable fact. But now,
the second position is that, even if this were not so, the Duke's
conclusion would not follow. This conclusion, it will be remembered, is,
that if incipient structures are useless, it necessarily follows that
natural selection can have had no part whatever in their inception. Now,
this is a conclusion which does not "necessarily" follow. Even if it be
granted that there are structures which in their first beginnings are
not of any use at all for any purpose, it is still possible that they
may owe their origin to natural selection--not indeed directly, but
indirectly. This possibility arises from the occurrence in nature of a
principle which has been called the Correlation of Growth.

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