Darwin himself has pointed out that the rudimentary wings of island
beetles, at first thought to be due to disuse, are mainly brought about
by natural selection--the best-winged beetles being most liable to be
blown out to sea. But he says that in birds of the oceanic islands "not
persecuted by any enemies, the reduction of their wings has probably
been caused by disuse." This explanation may be as fallacious as it is
acknowledged to have been in the case of the island beetles. According
to Darwin's own views, natural selection _must_ at least have played an
important part in reducing the wings; for he holds that "natural
selection is continually trying to economize every part of the
organization." He says: "If under changed conditions of life a
structure, before useful, becomes less useful, its diminution will be
favoured, for it will profit the individual not to have its nutriment
wasted in building up an useless structure.... Thus, as I believe,
natural selection will tend in the long run to reduce any part of the
organization, as soon as it becomes, through changed habits,
superfluous."[19] If, as Darwin powerfully urges (and he here ignores
his usual explanation), ostriches' wings are insufficient for flight in
consequence of the economy enforced by natural selection,[20] why may
not the reduced wings of the dodo, or the penguin, or the apteryx, or of
the Cursores generally, be wholly attributed to natural selection in
favour of economy of material and adaptation of parts to changed
conditions? The great principle of economy is continually at work
shaping organisms, as sculptors shape statues, by removing the
superfluous parts; and a mere glance at the forms of animals in general
will show that it is well-nigh as dominant and universal a principle as
is that of the positive development of useful parts. Other causes,
moreover besides actual economy, would favour shorter and more
convenient wings on oceanic islands. In the first place, birds that were
somewhat weak on the wing would be most likely to settle on an island
and stay there. Shortened wings would then become advantageous because
they would restrain fatal migratory tendencies or useless and perilous
flights in which the birds that flew furthest would be most often
carried away by storms and adverse winds. Reduced wings would keep the
birds near the shelter and the food afforded by the island and its
neighbourhood, and in some cases would become adapted to act as fins or
flappers for swimming under water in pursuit of fish.
The reduced size of the wings of these island birds is paralleled by the
remarkable thinness, &c., of the shell of the "gigantic land-tortoise"
of the Galapagos Islands. The changes seen in the carapace can hardly
have been brought about by the inherited effects of special disuse. Why
then should not the reduction of equally useless, more wasteful, and
perhaps positively dangerous wings be also due to an economy which has
become advantageous to bird and reptile alike through the absence of the
mammalian rivals whose places they are evidently being modified to fill?
The _complete_ loss of the wings in neuter ants and termites can
scarcely be due to the inherited effects of disuse; and as natural
selection has abolished these wings in spite of the opposition of
use-inheritance, it must clearly be fully competent to reduce wings
without its aid. In considering the rudimentary wings of the apteryx,
or of the moa, emu, ostrich, &c., we must not forget the frequent or
occasional occurrence of hard seasons, and times of drought and famine,
when Nature eliminates redundant, wasteful, and ill-adapted organisms in
so severe and wholesale a fashion. Where enemies are absent there would
be unrestrained multiplication, and this would greatly increase the
severity of the competition for food, and so hasten the elimination of
disused and useless parts.