120 THE ORIGIN OF SPECIES 



adjoining part being reduced by the 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 selec- 

 tion is continually trying to economize every part of the organiza- 

 tion. If, under changed conditions of life, a structure, before use- 

 ful, becomes less useful, its diminution will be favored, 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 analogous instances could be given: namely, that when a 

 cirripede is parasitic within another cirripede, and is thus pro- 

 tected, 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 cir- 

 ripedes 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 ex- 

 posed, 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 organization, as soon as it becomes, 

 through changed habits, superfluous, without by any means caus- 

 ing some other part to be largely developed in a corresponding 

 degree. And conversely, that natural selection may perfectly well 

 succeed in largely developing an organ without requiring as a 

 necessary compensation the reduction of some adjoining part. 



MULTIPLE, RUDIMENTARY, AND LOWLY ORGANIZED 

 STRUCTURES ARE VARIABLE 



It seems to be a rule, as remarked by Is. Geoffroy Saint-Hilaire, 

 both with varieties and species, that when any part or organ is 

 repeated many times in the same individual (as the vertebrae in 

 snakes, and the stamens in polyandrous flowers) the number is 

 variable; whereas the same part or organ, when it occurs in lesser 



