is generally accompanied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by this same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the
excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying
to economise in every part of the organisation. If under
changed conditions of life a structure before useful becomes less
useful, any 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 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 antenn . 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
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