Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring
from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species
inhabiting the same confined locality. No one supposes that all the
individuals of the same species are cast in the very same mould. These
individual differences are highly important for us, as they afford
materials for natural selection to accumulate, in the same manner as
man can accumulate in any given direction individual differences in
his domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could show
by a long catalogue of facts, that parts which must be called
important, whether viewed under a physiological or classificatory
point of view, sometimes vary in the individuals of the same species.
I am convinced that the most experienced naturalist would be surprised
at the number of the cases of variability, even in important parts of
structure, which he could collect on good authority, as I have
collected, during a course of years. It should be remembered that
systematists are far from pleased at finding variability in important
characters, and that there are not many men who will laboriously
examine internal and important organs, and compare them in many
specimens of the same species.
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