part to be largely developed in a corresponding degree. 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 vertebr 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. Natural selection, it should never be forgotten, can
act on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to
the general subject of rudimentary and aborted organs; and I will here
only add that their variability seems to be owing to their uselessness,
and therefore to natural selection having no power to check deviations
in their structure. Thus
rudimentary parts are left to the free play of the various laws of
growth, to the effects of long-continued disuse, and to the tendency to
reversion.
_A part developed in any species in an extraordinary degree or manner,
117