der
domestication.

We find that _all_ the changes are in the direction of shorter and
thicker bones--a tendency which must be largely dependent upon the
suspension of the rigorous elimination which keeps the bones of the
wild duck _long and light_. The used leg-bones and the disused
wing-bones have alike been shortened and thickened, though in different
proportions. Natural or artificial selection might easily thicken legs
without lengthening them, or shorten wings without eliminating strong
heavy bones, but it can hardly be contended that use-inheritance has
acted in such conflicting ways. The thickening of the wing-bones has
actually more than kept pace with any increase of weight in the
skeleton, in spite of the effect of individual disuse and of the alleged
cumulative effect of ancestral disuse for hundreds of generations. The
case of the duck deserves special attention as a crucial one, if only
from the fact that in this instance, and in this instance only, has
Darwin given the weights of the skeletons, thus furnishing the means for
a closer examination of his details than is usually possible.

If we ignore such factors as selection, panmixia, correlation, and the
effects of use and disuse during lifetime, and still regard the case of
the domestic duck as a valid proof of the inheritance of the effects of
use and disuse, we must also accept it as an equally valid proof that
the effects of use and disuse are _not_ inherited. Nay, we may even have
to admit that, in two points out of four, the _inherited_ effect of use
and disuse on successive generations is exactly opposite to the
immediate effect on the individual.

Among fowls the wing-bones have lost much in weight but little or
nothing in length--which is the reverse of what has occurred in ducks,
although disuse is alleged to be the common cause in both cases. Some of
the fowls which fly least have their wing-bones as long as ever. In the
case of the Silk and Frizzled fowls--ancient breeds which "cannot fly at
all"--and in that of the Cochins, which "can hardly fly up to a low
perch," Darwin observes "how truly the proportions of an organ may be
inherited although not fully exercised during many generations."[26] In
four out of twelve breeds the wing-bones had become slightly heavier
relatively to the leg-bones. Do not these facts tend to show that the
changes in fowls' wings are due to fluctuating variability and selective
influences rather than to a genera