as: 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 by the parasitic habits of the Proteolepas,
though effected by slow steps, 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 individual Proteolepas would have
a better chance of supporting itself, by less nutriment being wasted in
developing a structure now become useless.

Thus, as I believe, natural selection will always succeed in the long
run in reducing and saving every part of the organisation, as soon as it
is rendered 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 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 th