132 MULTIPLE AND RUDIMENTARY. 



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 economize every 

 part of the organization. If, under changed conditions of 

 life, a structure, before useful, becomes less useful, its dimi- 

 nution 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 anal- 

 ogous instances could be given : namely, that when a cirri- 

 pede 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 cirripedes consists of the three highly 

 important anterior segments of the head enormously devel- 

 oped, 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 exposed, each would have a better chance of sup- 

 porting 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 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 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 numbers, is constant. The 



