148 
LAWS OF YARIATIOi^’. 
Chap. V. 
changed conditions of life a structure before useful be¬ 
comes 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 extraordi¬ 
nary 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 ante¬ 
rior 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 ren¬ 
dered superfluous by the parasitic habits of the Proteo¬ 
lepas, 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 ex¬ 
posed, 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 suc¬ 
ceed 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, con¬ 
versely, that natural selection may perfectly well suc¬ 
ceed in largely developing any organ, without requiring 
as a necessary compensation the reduction of some ad¬ 
joining part. 
