t wasted in building up a useless structure. I can
thus only understand a fact with which I was much struck when examining
cirripedes, and of which many other instances could be given: namely,
that when a cirripede is parasitic within another cirripede and is thus
protected, it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly extraordinary manner
with the Proteolepas: for the carapace in all other cirripedes consists
of the three highly important anterior segments of the head enormously
developed, and furnished with great nerves and muscles; but in the
parasitic and protected Proteolepas, the whole anterior part of the
head is reduced to the merest rudiment attached to the bases of the
prehensile antennae. Now the saving of a large and complex structure,
when rendered superfluous, would be a decided advantage to each
successive individual of the species; for in the struggle for life
to which every animal is exposed, each would have a better chance of
supporting itself, by less nutriment being wasted.

Thus, as I believe, natural selection will tend in the long run to
reduce any part of the organisation, as soon as it becomes, through
changed habits, superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And conversely,
that natural selection may perfectly well succeed in largely developing
an organ without requiring as a necessary compensation the reduction of
some adjoining part.

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 particula