It has been so generally
the practice to speak of variation as something exceptional and
comparatively rare--as an abnormal deviation from the uniformity and
stability of the characters of a species--and so few even among
naturalists have ever compared, accurately, considerable numbers of
individuals, that the conception of variability as a general
characteristic of all dominant and widespread species, large in its
amount and affecting, not a few, but considerable masses of the
individuals which make up the species, will be to many entirely new.
Equally important is the fact that the variability extends to every
organ and every structure, external and internal; while perhaps most
important of all is the independent variability of these several parts,
each one varying without any constant or even usual dependence on, or
correlation with, other parts. No doubt there is some such correlation
in the differences that exist between species and species--more
developed wings usually accompanying smaller feet and _vice versa_--but
this is, generally, a useful adaptation which has been brought about by
natural selection, and does not apply to the individual variability
which occurs within the species.
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