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. Thus favorable variations tend to
increase and become more marked from generation to generation.

Now it has always been known that breeders could produce a race of
markedly peculiar form or characteristics by selecting the
individuals possessing this quality in the highest degree and
breeding only from these. The breeder depends upon heredity,
variation, and his selection of the individuals from which to breed.
Similarly in nature new species have arisen through heredity,
variation, and a selection according to the laws of nature of those
varying in conformity with their environment. And this Mr. Darwin
called natural, in contrast with the breeder's artificial,
"selection," arising from the "struggle for existence," and
resulting in what Mr. Spencer has called the "survival of the
fittest."