organs, seem to have been more potent in their effects. Homologous
parts tend to vary in the same way, and homologous parts tend to
cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed,
perhaps it tends to draw nourishment from the adjoining parts; and
every part of the structure which can be saved without detriment to the
individual, will be saved. Changes of structure at an early age will
generally affect parts subsequently developed; and there are very many
other correlations of growth, the nature of which we are utterly unable
to understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialised to
any particular function, so that their modifications have not been
closely checked by natural selection. It is probably from this same
cause that organic beings low in the scale of nature are more variable
than those which have their whole organisation more specialised, and
are higher in the scale. Rudimentary organs, from being useless, will
be disregarded by natural selection, and hence probably are variable.
Specific characters that is, the characters which have come to differ
since the several species of the same genus branched off from a common
parent are more variable than generic characters, or those which have
long been inherited, and have not differed within this same period. 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 any 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 there, on an average, we now find most
varieties or incipient species. 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
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
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