Since then evidence of this nature has been accumulating,
and we are, therefore, now in a far better position to appreciate the
facilities for natural selection, in this respect, than was Mr. Darwin
himself.
Another objection of a similar nature is, that the chances are immensely
against the right variation or combination of variations occurring just
when required; and further, that no variation can be perpetuated that is
not accompanied by several concomitant variations of dependent
parts--greater length of a wing in a bird, for example, would be of
little use if unaccompanied by increased volume or contractility of the
muscles which move it. This objection seemed a very strong one so long
as it was supposed that variations occurred singly and at considerable
intervals; but it ceases to have any weight now we know that they occur
simultaneously in various parts of the organism, and also in a large
proportion of the individuals which make up the species. A considerable
number of individuals will, therefore, every year possess the required
combination of characters; and it may also be considered probable that
when the two characters are such that they always _act_ together, there
will be such a correlation between them that they will frequently _vary_
together.
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