apace 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. {149}

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 Prof. 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