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 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 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.
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