CHAPTER V. LAWS OF VARIATION.
6. MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with
varieties and species, that when any part or organ is repeated many times
in 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 as well as some botanists, have further remarked that multiple parts
are extremely liable to vary in structure. As "vegetative repetition," to
use Professor Owen's expression, is a sign of low organisation; the
foregoing statements accord with the common opinion of naturalists, that
beings which stand low in the scale of nature are more variable than those
which are higher. I presume that lowness here 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 not have preserved or rejected each little deviation of
form so carefully as when the part has to serve for some one special
purpose. 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 purpose must
be of some particular shape. Natural selection, it should never be
forgotten, can act solely through and for the advantage of each being.
Rudimentary parts, as is generally admitted, are apt to be highly variable.
We shall have to recur to this subject; and I will here only add that their
variability seems to result from their uselessness, and consequently from
natural selection having had no power to check deviations in their