The reduced wings and enlarged legs of domesticated ducks and fowls are
attributed by Darwin and Spencer to the inheritance of the effects of
use and disuse. But the inference by no means follows. Natural selection
would usually favour these adaptive changes, and they would also have
been aided by an artificial selection which is often unconscious or
indirect. Birds with diminished power of flight would be less difficult
to keep and manage, and in preserving and multiplying such birds man
would be unconsciously bringing about structural changes which would
easily be regarded as effects of use and disuse. "About eighteen
centuries ago Columella and Varro speak of the necessity of keeping
ducks in netted enclosures like other wild fowl, so that at this period
there was danger of their flying away."[22] Is it not probable that the
best fliers would escape most frequently, or would pine most if kept
confined? On the other hand, birds with lessened powers of flight would
not be eliminated as under natural conditions, but would be favoured;
and natural selection, together with artificial selection of the most
flourishing birds, would thicken and strengthen the legs to meet
increased demands upon them.
The diminution of the duck's wing is not great even in the birds that
"never fly," and from this we must deduct the direct effect of disuse on
the individual during its lifetime. As Weismann suggests, the
_inherited_ portion of the change could only be ascertained by comparing
the bones, &c., of wild and tame ducks _similarly reared_. If individual
disuse diminished the weight of the duck's wing-bones by 9 per cent.
there would be nothing left to account for.
I suspect that investigation would reveal anomalies inconsistent with
the theory of use-inheritance. Thus according to Darwin's tables of
comparative weights and measurements[23] the leg-bones of the Penguin
duck have slightly diminished in length, although they have increased 39
per cent. in weight. Relatively to the weight of the skeleton, the
leg-bones have shortened in the tame breeds of ducks by over 5 per cent.
(and in two breeds by over 8 per cent.) although they have increased
more than 28 per cent. in proportional weight.[24] How can increased use
simultaneously shorten and thicken these bones? If the relative
shortening is attributed to a heavier skeleton, then the apparently
reduced weight of the wing-bones is fully accounted for by the same
circumstance, and disuse has had no inherited effect.
Another strange circumstance is that the wing-bones have diminished _in
length only_. The shortening is about 6 per cent. more than in the
shortened legs, and it amounts to 11 per cent. as compared with the
weight of the skeleton. Such a shortening should represent a reduction
of 29 per cent. in weight, whereas the actual reduction in the weight of
the wing-bones relatively to the weight of the skeleton is only 9 per
cent. even in the breeds that never fly. Independently of shortening,
the disused wing-bones have actually thickened or increased in weight.
In the Aylesbury duck the disproportion caused by these conflicting
changes is so great that the wing-bones are 47 per cent. heavier than
they should be if their weight had varied proportionally with their
length.[25] The reduction in weight on which Darwin relies seems to be
entirely due to the shortening, and this shortening appears to be
irrelevant to disuse, since the wings of the Call duck are similarly
shortened in their proportions by 12 per cent., although this bird
habitually flies to such an extent that Darwin partly attributes the
greatly increased weight of its wing-bones to increased use under
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 general law whereby the effects of disuse
are cumulatively inherited?