classing genera. I believe this
explanation is partly, yet only indirectly, true; I shall, however, have
to 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 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
probable that they should var