shment from
the adjoining parts; and every part of the structure which can be saved
without detriment will be saved. Changes of structure at an early age
may affect parts subsequently developed; and many cases of correlated
variation, the nature of which we are unable to understand, undoubtedly
occur. Multiple parts are variable in number and in structure, perhaps
arising from such parts not having been closely specialised for any
particular function, so that their modifications have not been closely
checked by natural selection. It follows probably from this same cause,
that organic beings low in the scale are more variable than those
standing higher in the scale, and which have their whole organisation
more specialised. Rudimentary organs, from being useless, are not
regulated by natural selection, and hence 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 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 ov