But there is another consideration that seems to show that
this coincident variation is not essential. All animals in a state of
nature are kept, by the constant struggle for existence and the survival
of the fittest, in such a state of perfect health and usually
superabundant vigour, that in all ordinary circumstances they possess a
surplus power in every important organ--a surplus only drawn upon in
cases of the direst necessity when their very existence is at stake. It
follows, therefore, that _any_ additional power given to one of the
component parts of an organ must be useful--an increase, for example,
either in the wing muscles or in the form or length of the wing might
give _some_ increased powers of flight; and thus alternate
variations--in one generation in the muscles, in another generation in
the wing itself--might be as effective in permanently improving the
powers of flight as coincident variations at longer intervals. On either
supposition, however, this objection appears to have little weight if we
take into consideration the large amount of coincident variability that
has been shown to exist.
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