n 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. 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 i