with such parts slightly less
developed, would in the course of ages produce in organic beings under
domesticity races with such parts abortive. We have every reason to
believe that every part and organ in an individual becomes fully
developed only with exercise of its functions; that it becomes developed
in a somewhat lesser degree with less exercise; and if forcibly
precluded from all action, such part will often become atrophied. Every
peculiarity, let it be remembered, tends, especially where both parents
have it, to be inherited. The less power of flight in the common duck
compared with the wild, must be partly attributed to disuse{501} during
successive generations, and as the wing is properly adapted to flight,
we must consider our domestic duck in the first stage towards the state
of the Apteryx, in which the wings are so curiously abortive. Some
naturalists have attributed (and possibly with truth) the falling ears
so characteristic of most domestic dogs, some rabbits, oxen, cats,
goats, horses, &c., &c., as the effects of the lesser use of the muscles
of these flexible parts during successive generations of inactive life;
and muscles, which cannot perform their functions, must be considered
verging towards abortion. In flowers, again, we see the gradual abortion
during successive seedlings (though this is more properly a conversion)
of stamens into imperfect petals, and finally into perfect petals. When
the eye is blinded in early life the optic nerve sometimes becomes
atrophied; may we not believe that where this organ, as is the case with
the subterranean mole-like Tuco-tuco <_Ctenomys_>{502}, is frequently
impaired and lost, that in the course of generations the whole organ
might become abortive, as it normally is in some burrowing quadrupeds
having nearly similar habits with the Tuco-tuco?

{501} _Origin_, Ed. i. p. 11, vi. p. 13, where drooping-ears of
domestic animals are also given.

{502} _Origin_, Ed. i. p. 137, vi. p. 170.

In as far then as it is admitted as probable that the effects of disuse
(together with occasional true and sudden abortions during the embryonic
period) would cause a part to be less developed, and finally to become
abortive and useless; then during the infinitely numerous changes of
habits in the many descendants from a common stock, we might fairly have
expected that cases of organs becom<ing> abortive would have been numerous.
The preservation of the stump of t