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.
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