only used
for classing genera. I believe this explanation is partly, yet only
indirectly, true; I shall, however, have to {155} return to this subject in
our chapter on Classification. It would be almost superfluous to adduce
evidence in support of the above statement, that specific characters are
more variable than generic; but I have repeatedly noticed in works on
natural history, that when an author has remarked with surprise that some
_important_ organ or part, which is generally very constant throughout
large groups of species, has _differed_ considerably in closely-allied
species, that it has, also, been _variable_ in the individuals of some of
the 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 seems to entertain no doubt, that the more an organ
normally differs in the different species of the same group, the more
subject it is to individual anomalies.

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 of species being
only strongly marked and fixed varieties, we might surely expect to find
them still often 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
the species of some other genus, are called generic characters; and these
characters in common I attribute to {156} inheritance from a common
progenitor, for it can rarely have happened that natural selection will
have modified several 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 a remote period, since that period when
the 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