And,
conversely, that natural selection may perfectly well succeed in
largely developing any organ, without requiring as a necessary
compensation the reduction of some adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
in varieties and in species, that when any part or organ is repeated
many times in the structure of the same individual (as the vertebrae
in snakes, and the stamens in polyandrous flowers) the number is
variable; whereas the number of the same part or organ, when it occurs
in lesser numbers, is constant. The same author and some botanists
have further remarked that multiple parts are also very liable to
variation in structure. Inasmuch as this "vegetative repetition," to
use Professor Owen's expression, seems to be a sign of low
organisation; the foregoing remark seems connected with the very
general opinion of naturalists, that beings low in the scale of nature
are more variable than those which are higher. I presume that lowness
in this case means that the several parts of the organisation have
been but little specialised for particular functions; and as long as
the same part has to perform diversified work, we can perhaps see why
it should remain variable, that is, why natural selection should have
preserved or rejected each little deviation of form less carefully
than when the part has to serve for one special purpose alone. In the
same way that a knife which has to cut all sorts of things may be of
almost any shape; whilst a tool for some particular object had better
be of some particular shape.
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