I should never have 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; I should have
expected that changes of this nature could have been effected only by
slow degrees: yet quite recently Mr. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be
compared to the irregular branching of the stem of a tree. This
philosophical naturalist, I may add, has also quite recently shown
that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank
that character as important (as some few naturalists have honestly
confessed) which does not vary; and, under this point of view, no
instance of an important part varying will ever be found: but under
any other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seems
to me extremely perplexing: I refer to those genera which have
sometimes been called "protean" or "polymorphic," in which the species
present an inordinate amount of variation; and hardly two naturalists
can agree which forms to rank as species and which as varieties. We
may instance Rubus, Rosa, and Hieracium amongst plants, several genera
of insects, and several genera of Brachiopod shells.
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