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 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 within the
period of the branching off of the species from a common progenitor, it
is probable that they should still often be in some degree variable, at
least more variable than those parts of the organisation which have for
a very long period remained constant.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been accumulated by
sexual selection, which
is less rigid in its action than ordinary selection, as it does not
entail death, but only gives fewer offspring to the less favoured
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