Even birds, slowly as they
increase, produce in a lifetime probably at least from twelve to
twenty eggs. Now let us suppose that all these eggs developed, and
all the birds lived out their normal period of life, and reproduced
at the same rate. After not many centuries there would not be
standing room on the globe for the descendants of a single pair.
Again, of the one hundred eggs of an insect let us suppose that only
sixty develop into the first larval, caterpillar, stage. Of these
sixty, the number of members of the species remaining constant, only
two will survive. The other fifty-eight die--of starvation,
parasites, or other enemies, or from inclement weather. Now which
two of all shall survive? Those naturally best able to escape their
enemies or to resist unfavorable influences; in a word, those best
suited to their conditions, or, to use Mr. Darwin's words,
"conformed to their environment."
Now if any individual has varied so as to possess some peculiarity
which enables it even in slight degree to better escape its enemies
or to resist unfavorable conditions, those of its descendants who
inherit most markedly this peculiar quality or variation will be the
most likely to escape, those without it to perish. If a form varies
unfavorably, becomes for instance more conspicuous to its enemies,
it will almost certainly perish.
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