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
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 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 are possessed by all the species; 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 slight 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
their great difference in 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 being the descendants of 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 having been adapted for secondary sexual, and for
ordinary purposes.
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