Now variations ascribed mainly to use
and disuse can be supposed capable of being accumulated, for use and
disuse are fairly constant for long periods among the individuals of
the same species, and often over large areas; moreover, conditions
of existence involving changes of habit, and thus of organization,
come for the most part gradually; so that time is given during which
the organism can endeavour to adapt itself in the requisite
respects, instead of being shocked out of existence by too sudden
change. Variations, on the other hand, that are ascribed to mere
chance cannot be supposed as likely to be accumulated, for chance is
notoriously inconstant, and would not purvey the variations in
sufficiently unbroken succession, or in a sufficient number of
individuals, modified similarly in all the necessary correlations at
the same time and place to admit of their being accumulated. It is
vital therefore to the theory of evolution, as was early pointed out
by the late Professor Fleeming Jenkin and by Mr.
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