148 LAWS OF VAKIATION. 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. 



