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. 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 shown that the muscles in the larvae of
certain insects are far from uniform. Authors sometimes argue in a circle
when they state that important organs never vary; for these same authors
practically rank those parts as important (as some few naturalists have
honestly confessed) which do not vary; and, under this point of view, no
instance will ever be found of an important part varying; but under any
other point of view many instances assuredly can be given.

There is one point connected with individual differences which is extremely
perplexing: I refer to those genera which have been called "protean" or
"polymorphic," in which species present an inordinate amount of variation.
With respect to many of these forms, hardly two naturalists agree whether
to rank them as species or as varieties. We may instance Rubus, Rosa, and
Hieracium among plants, several genera of insects, and of Brachiopod
shells. In most polymorphic genera some of the species have fixed and
definite characters. Genera which are polymorphic in one country seem to
be, with a few exceptions, polymorphic in other countries, and likewise,