tatement, that ordinary specific characters
are more variable than generic; but with respect to important
characters, I have repeatedly noticed in works on natural history, that
when an author remarks with surprise that some important organ or part,
which is generally very constant throughout a large group of species,
DIFFERS considerably in closely-allied species, it is often VARIABLE
in the individuals of the same species. And this fact shows that a
character, which is generally of generic value, when it sinks in value
and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same kind
applies to monstrosities: at least Is. Geoffroy St. Hilaire apparently
entertains no doubt, that the more an organ normally differs in the
different species of the same group, the more subject it is to anomalies
in the individuals.

On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same
part in other independently created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view that species are only strongly marked and fixed varieties, we might
expect often to find them still continuing to vary in those parts of
their structure which have varied within a moderately recent period, and
which have thus come to differ. Or to state the case in another manner:
the points in which all the species of a genus resemble each other, and
in which they differ from allied genera, are called generic characters;
and these characters may be attributed to inheritance from a common
progenitor, for it can rarely have happened that natural selection will
have modified several distinct species, fitted to more or less widely
different habits, in exactly the same manner: and as these so-called
generic characters have been inherited from before the period when the
several species first branched off from their common progenitor, and
subsequently have not varied or come to differ in any degree, or only in
a slight degree, it is not probable that they should vary at the present
day. On the other hand, the points in which species differ from other
species of the same genus are called specific characters; and as these
specific characters have varied and come to differ since the period when
the sp