In these
remarks we have referred to special parts or organs being still variable,
because they have recently varied and thus come to differ; but we have also
seen in the second chapter that the same principle applies to the whole
individual; for in a district where many species of a genus are found--that
is, where there has been much former variation and differentiation, or
where the manufactory of new specific forms has been actively at work--in
that district and among these species, we now find, on an average, most
varieties. Secondary sexual characters are highly variable, and such
characters differ much in the species of the same group. Variability in
the same parts of the organisation has generally been taken advantage of in
giving secondary sexual differences to the two sexes of the same species,
and specific differences to the several species of the same genus. Any
part or organ developed to an extraordinary size or in an extraordinary
manner, in comparison with the same part or organ in the allied species,
must have gone through an extraordinary amount of modification since the
genus arose; and thus we can understand why it should often still be
variable in a much higher degree than other parts; for variation is a
long-continued and slow process, and natural selection will in such cases
not as yet have had time to overcome the tendency to further variability
and to reversion to a less modified state.
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