males. Whatever the cause may be of the variability of secondary sexual
characters, as they are highly variable, sexual selection will have had
a wide scope for action, and may thus readily have succeeded in giving
to the species of the same group a greater amount of difference in
their sexual characters, than in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of the
same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my list;
and as the differences in these cases are of a very unusual nature, the
relation can hardly be accidental. The same number of joints in the
tarsi is a character generally common to very large groups of beetles,
but in the Engid , as Westwood has remarked, the number varies greatly;
and the number likewise differs in the two sexes of the same species:
again in fossorial hymenoptera, the manner of neuration of the wings is
a character of the highest importance, because common to large groups;
but in certain genera the neuration differs in the different species,
and likewise in the two sexes of the same species. This relation has a
clear meaning on my view of the subject: I look at all the species of
the same genus as having as certainly descended from the same
progenitor, as have the two sexes of any one of the species.
Consequently, whatever part of the structure of the common progenitor,
or of its early descendants, became variable; variations of this part
would it is highly probable, be taken advantage of by natural and
sexual selection, in
order to fit the several species to their several places in the economy
of nature, and likewise to fit the two sexes of the same species to
each other, or to fit the males and females to different habits of
life, or the males to struggle with other males for the possession of
the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common; that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in
its congeners; and the not great degree of variability in a part,
however extraordinarily it may be developed, if it be common to a whole
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