]40 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 econo- 

 mize every part of the organization. If under changed 

 conditions of life a structure, before useful, 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 cirripedo is parasitic within another cirripede 

 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 extraordinary manner with the Proteo- 

 lepas: 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 Proteole- 

 pas, 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 struc- 

 ture, when rendered superfluous, would be a decided ad- 

 vantage 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 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 causing some other "part to be largely devef- 

 oped in a corresponding degree. And conversely, that 

 natural selection may perfectly well succeed in largely de- 

 veloping an organ without requiring as a necessary com- 

 pensation the reduction of some adjoining part. 



MULTIPLE, KUDIMEKTARY, AND LOtt'LT-OEGANIZED STRUC- 

 TURES ARE VARIABLE. 



It seems to be a rule, as remarked by Is. Geoffrey St. 

 Hilaire, both with varieties and species, that when any part 

 or organ is repeated many times in the same individual (as 



