HE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY
NATURAL SELECTION.

From the facts alluded to in the first chapter, I think there can be
no doubt that use in our domestic animals has strengthened and enlarged
certain parts, and disuse diminished them; and that such modifications
are inherited. Under free nature we have no standard of comparison by
which to judge of the effects of long-continued use or disuse, for we
know not the parent-forms; but many animals possess structures which
can be best explained by the effects of disuse. As Professor Owen has
remarked, there is no greater anomaly in nature than a bird that cannot
fly; yet there are several in this state. The logger-headed duck of
South America can only flap along the surface of the water, and has its
wings in nearly the same condition as the domestic Aylesbury duck: it is
a remarkable fact that the young birds, according to Mr. Cunningham, can
fly, while the adults have lost this power. As the larger ground-feeding
birds seldom take flight except to escape danger, it is probable that
the nearly wingless condition of several birds, now inhabiting or which
lately inhabited several oceanic islands, tenanted by no beasts of prey,
has been caused by disuse. The ostrich indeed inhabits continents, and
is exposed to danger from which it cannot escape by flight, but it
can defend itself, by kicking its enemies, as efficiently as many
quadrupeds. We may believe that the progenitor of the ostrich genus had
habits like those of the bustard, and that, as the size and weight of
its body were increased during successive generations, its legs were
used more and its wings less, until they became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off;
he examined seventeen specimens in his own collection, and not one had
even a relic left. In the Onites apelles the tarsi are so habitually
lost that the insect has been described as not having them. In some
other genera they are present, but in a rudimentary condition. In the
Ateuchus or sacred beetle of the Egyptians, they are totally deficient.
The evidence that accidental mutilations can be inherited is at present
not decisive; but the remarkable cases observed by Brown-Sequard in
guinea-pigs, of the inherited effects of operations, should make us
cautious in denying this tendency. Hence, it will perhaps be sa