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 group of species;--that the great variability
of secondary sexual characters, and the great amount of difference in
these same characters between closely allied species;--that secondary
sexual and ordinary specific differences are generally displayed in
the same parts of the organisation,--are all principles closely
connected together. All being mainly due to the species of the same
group having descended from a common progenitor, from whom they have
inherited much in common,--to parts which have recently and largely
varied being more likely still to go on varying than parts which have
long been inherited and have not varied,--to natural selection having
more or less completely, according to the lapse of time, overmastered
the tendency to reversion and to further variability,--to sexual
selection being less rigid than ordinary selection,--and to variations
in the same parts having been accumulated by natural and sexual
selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.
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