though it is still obvious that they
were once organs of flight; and in these cases we certainly require some
other causes than those which have reduced the wings of our domestic
fowls. One such cause may have been of the same nature as that which has
been so efficient in reducing the wings of the insects of oceanic
islands--the destruction of those which, during the occasional use of
their wings, were carried out to sea. This form of natural selection may
well have acted in the case of birds whose powers of flight were
already somewhat reduced, and to whom, there being no enemies to escape
from, their use was only a source of danger. We may thus, perhaps,
account for the fact that many of these birds retain small but useless
wings with which they never fly; for, the wings having been reduced to
this functionless condition, no power could reduce them further except
correlation of growth or economy of nutrition, causes which only rarely
come into play.

The complete loss of eyes in some cave animals may, perhaps, be
explained in a somewhat similar way. Whenever, owing to the total
darkness, they became useless, they might also become injurious, on
account of their delicacy of organisation and liability to accidents and
disease; in which case natural selection would begin to act to reduce,
and finally abort them; and this explains why, in some cases, the
rudimentary eye remains, although completely covered by a protective
outer skin. Whales, like moas and cassowaries, carry us back to a remote
past, of whose conditions we know too little for safe speculation. We
are quite ignorant of the ancestral forms of either of these groups, and
are therefore without the materials needful for determining the steps by
which the change took place, or the causes which brought it about.[200]

On a review of the various examples that have been given by Mr. Darwin
and others of organs that have been reduced or aborted, there seems too
much diversity in the results for all to be due to so direct and uniform
a cause as the individual effects of disuse accumulated by heredity. For
if that were the only or chief efficient cause, and a cause capable of
producing a decided effect during the comparatively short period of the
existence of animals in a state of domestication, we should expect to
find that, in wild species, all unused parts or organs had been reduced
to the smallest rudiments, or had wholly disappeared. Instead of this we
find vari