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. In most polymorphic genera
some of the species have fixed and definite characters. Genera which
are polymorphic in one country seem to be, with some few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts seem to be very
perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that we
see in these polymorphic genera variations in points of structure which
are of no service or disservice to the species, and which consequently
have not been seized on and rendered definite by natural selection, as
hereafter will be explained.

Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of
these doubtful and closely-allied forms have permanently retained their
characters in their own country for a long time; for as long, as far as
we know, as have good and true species. Practically, when a naturalist
can unite two forms together by others having intermediate characters,
he treats the one as a variety of the other, ranking the most common,
but sometimes the one first described, as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes occur in deciding whether or not to rank one
form as a variety of another, even when they are closely connected by
intermediate links; nor will the commonly-assumed hybrid nature of the
intermediate links always remove the difficulty. In