CHAPTER V. LAWS OF VARIATION.
11. SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of
a hundred can we pretend to assign any reason why this or that part has
varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between species
of the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and
these may become strongly marked in the course of time, though we have not
sufficient evidence on this head. Habit in producing constitutional
peculiarities, and use in strengthening, and disuse in weakening and
diminishing organs, appear in many cases to have been potent in their
effects. Homologous parts tend to vary in the same manner, 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 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 overcome the tendency to further variability
and to reversion to a less modified state. But when a species with an
extraordinarily developed organ has become the parent of many modified
descendants--which on our view must be a very slow process, requiring a
long lapse of time--in this case, natural selection has succeeded in giving
a fixed character to the organ, in however extraordinary a manner it may
have been developed. Species inheriting nearly the same constitution from
a common parent, and exposed to similar influences, naturally tend to
present analogous variations, or these same species may occasionally revert
to some of the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous
variation, such modifications will add to the beautiful and harmonious
diversity of nature.
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