LAWS OF VARIATION 209 



it loses more or less completely its own shell or carapace. 

 This is the case with the male Ibla, and in a truly ex- 

 traordinary manner with the Proteolepas: for the carapace 

 in all other cirripeds consists of the three highly-impor- 

 tant 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 at- 

 tached to the bases of the prehensile antennas. Now the 

 saving of a large and complex structure, when rendered 

 superfluous, would be a decided advantage to each suc- 

 cessive individual of the species; for in the struggle for 

 life to which every anima\ is exposed each would have 

 a better chance of supporting itself, by less nutriment 

 being wasted. 



Thus, as I believe, natural sslection will tend in the 

 long run to reduce any part of the organization, as soon 

 as it becomes, through changed habits, superfluous, with- 

 out 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 com- 

 pensation 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 

 St.-Hilaire, both with varieties and species, that when 

 any part or organ is repeated many times in the same 

 individual (as the vertebras in snakes, and the stamens 

 in polyandrous flowers) the number is variable; whereas 

 the same part or organ, when it occurs in lesser num- 



