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.
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