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
time to overcome the tendency to further variability and to
reversion to a less modified state