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; {169} 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 de