These individual differences are of the highest importance for us, for they
are often inherited, as must be familiar to every one; and they thus afford
materials for natural selection to act on and accumulate, in the same
manner as man accumulates 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 being 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. It would never have been
expected that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species; it
might have been thought that changes of this nature could have been
effected only by slow degrees; yet Sir J.
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