The shortening of the sternum in pigeons is attributed to disuse of the
flight muscles attached to it. The bone is only shortened by a third of
an inch, but this represents a very remarkable reduction in proportional
length, which Darwin estimates at from one-seventh to one-eighth, or
over 13 per cent. This marked reduction, too, quite unlike the slight
reduction of the wing-bones to which the other ends of the muscles are
attached, was universal in the eleven specimens measured by Darwin; and
the bone, though acknowledged to have been modified by artificial
selection in some breeds, is not so open to observation as wings or
legs. Even, however, if this relative shortening of the sternum remained
otherwise inexplicable, it might still be as irrelevant to use and
disuse as is the fact that "many breeds" of fancy pigeons have lost a
rib, having only seven where the ancestral rock-pigeon has eight.[30]
But the excessive reduction in the sternum is far from being
inexplicable. In the first place Darwin has somewhat over-estimated it.
Instead of comparing the deficiency of length with the increased length
which _should_ have been acquired (since the pigeons have increased in
average size) he compares it with the length of the breast-bone in the
rock-pigeon.[31] By this method if a pigeon had doubled in dimensions
while its breast-bone remained unaltered, the reduction would be put
down as 100 per cent., whereas obviously the true reduction would be
one-half, or 50 per cent. of what the bone _should be_. Avoiding this
error and a minor fallacy besides, a sound estimate reduces the supposed
reduction of 13 or 14 per cent. to one of 11.7 per cent., which is still
of course a considerable diminution.
Part of this reduction must be due to the direct effect of disuse during
the lifetime of the individual. Another and perhaps very considerable
part of the relative change must be attributed to the lengthening of the
neck or body by artificial selection, or to other modifications of
shape and proportion effected directly or indirectly by the same
cause.[32] The reduction is greatest in the Pouter (18-1/2 per cent.)
and in the Pied Scanderoon (17-1/2 per cent.). In the former the body
has been greatly elongated by artificial selection and three or four
additional vertebrae have been acquired in the hinder part of the
body.[33] In the latter a long neck increases the length of the bird,
and so causes, or helps to cause, the relative shortening of the
breast-bone. In the English Carrier--which experiences the effects of
disuse, as it is too valuable to be flown--the relative reduction of 11
per cent. is apparently more than accounted for by the "elongated
neck." The Dragon also has a long neck. In the Pouter, although the
breast-bone has been shortened by 18-1/2 per cent. relatively to the
length of the body, it has _lengthened_ by 20 per cent. relatively to
the _bulk_ of the body.[34] Darwin forgot to ask whether allowance must
not be made for a frequent, or perhaps general, elongation of the neck
and the hinder part of the body, and the relative shortening or the
throwing forward of the central portion containing the ribs (frequently
one less in number) and the sternum. The whole body of the pigeon is so
much under the control of artificial selection, that every precaution
must be taken to guard against such possible sources of error.[35]
Under domestication there would be a suspension of the previous
elimination of reduced breast-bones by natural selection (Weismann's
panmixia), and a diminution of the parts concerned in flying might even
be favoured, as lessened powers of _continuous_ flight would prevent
pigeons from straying too far, and would fit them for domestication or
confinement. Such causes might reduce some of the less observed parts
affected by flying, while still leaving the wing of full size for
occasional flight, or to suit the requirements of the pigeon-fanciers. A
change might thus be commenced like that seen in the rudimentary keel of
the sternum in the owl-parrot of New Zealand, which has lost the power
of flight although still retaining fairly-developed wings.