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however slight, in its development, 1859 1860 1861 1866 1869
OMIT 1872

seized on by natural selection, 1859 1860 1861 1866 1869
favoured, 1872

by the parasitic habits 1859 1860 1861 1866 1869
would be a decided advantage to each successive individual 1872

Proteolepas, though effected by slow steps, would be a decided advantage to each successive individual of the 1859 1860 1861 1866 1869
OMIT 1872

in reducing and saving every 1859 1860 1861 1866 1869
to reduce any 1872

is rendered 1859 1860 1861 1866 1869
becomes, through changed habits, 1872

superfluous, 1859 1860 1861 1872
by changed habits of life superfluous, 1866 1869

changed conditions of life a
structure,
structure
before
useful,
useful
becomes less useful,
its
any
diminution
diminution,
however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many
analogous
other
instances could be given: namely, that when a cirripede is parasitic within another
cirripede and
and
is thus protected, it loses more or less completely
is
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 antennæ. Now the saving of a large and complex structure, when rendered
superfluous,
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
individual Proteolepas
would have a better chance of supporting itself, by less nutriment being
wasted.
wasted
in
in
developing
developing
a
a
structure
structure
now
now
become
become
useless.
useless.
Thus, as I believe, natural selection will
tend
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
an
any
organ
organ,
without requiring as a necessary compensation the reduction of some adjoining part.