Let it be borne
in mind in what an endless number of strange peculiarities our
domestic productions, and, in a lesser degree, those under nature,
vary; and how strong the hereditary tendency is. Under domestication,
it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and
close-fitting are the mutual relations of all organic beings to each
other and to their physical conditions of life. Can it, then, be
thought improbable, seeing that variations useful to man have
undoubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should sometimes occur
in the course of thousands of generations? If such do occur, can we
doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that any
variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable variations and the rejection of
injurious variations, I call Natural Selection. Variations neither
useful nor injurious would not be affected by natural selection, and
would be left a fluctuating element, as perhaps we see in the species
called polymorphic.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate.
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