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 less weight on the direct action
of the surrounding conditions, than on a tendency to vary, due to causes of
which we are quite ignorant.
In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include natural
selection, for the conditions determine whether this or that variety shall
survive. But when man is the selecting agent, we clearly see that the two
elements of change are distinct; variability is in some manner excited, but
it is the will of man which accumulates the variations in certain
direction; and it is this latter agency which answers to the survival of
the fittest under nature.
EFFECTS OF THE 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.
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