If under changed conditions
of life a structure, before useful, becomes less useful, its diminution
will be favoured, for it will profit the individual not to have its
nutriment 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.
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