for it would appear that climate has some direct action on the hair of
our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same species
under the same conditions. Such facts show how indirectly the
conditions of life must act. Again, innumerable instances are known to
every naturalist of species keeping true, or not varying at all,
although living under the most opposite climates. Such considerations
as these incline me to lay very little weight on the direct action of
the conditions of life. Indirectly, as already remarked, they seem to
play an important part in affecting the reproductive system, and in
thus inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.
_Effects of Use and Disuse_. From the facts alluded to in the first
chapter, I think there can be little doubt that use in our domestic
animals strengthens and enlarges certain parts, and disuse diminishes
them; and that such modifications are inherited. Under free nature, we
can 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 have structures which can be 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. As the larger ground-feeding birds seldom
take flight except to escape danger, I believe that the nearly wingless
condition of several birds, which now inhabit or have lately inhabited
several oceanic islands, tenanted by no beast 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 by kicking it can
defend itself from enemies, as well as any of the smaller
quadrupeds. We may imagine that the early progenitor of the ostrich had
habits like those of a bustard, and that as natural selection increased
in successive generations the size and weight of its body, its legs
were used more, and its wings less, until they became incapable of
flight.
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