"It then," he says,
"occurred to me that these causes or their equivalents are continually
acting in the case of animals also; and as animals usually breed much
more rapidly than does mankind, the destruction every year from these
causes must be enormous in order to keep down the numbers of each
species, since they evidently do not increase regularly from year to
year, as otherwise the world would have been densely crowded with those
that breed most quickly.... Then it suddenly flashed upon me that this
self-acting process would necessarily _improve the race_, because in
every generation the inferior would inevitably be killed off and the
superior would remain--that is, the _fittest would survive_. Then at
once I seemed to see the whole effect of this, that when changes of land
and sea, or of climate, or of food-supply, or of enemies occurred--and
we know that such changes have always been taking place--and considering
the amount of individual variation that my experience as a collector had
shown me to exist, then it followed that all the changes necessary for
the adaptation of the species to the changing conditions would be
brought about; and as great changes in the environment are always slow,
there would be ample time for the change to be effected by the survival
of the best fitted in every generation.
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