COMPENSATION AND ECONOMY OF GROWTH. 183 



economise every part of trie organisation. If under 

 changed conditions of life a structure, before useful, 

 becomes less useful, its diminution will be favoured, 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 ana- 

 logous instances could be given : namely, that when 

 a cirripede 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 

 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 

 iDdividual 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 organisation, 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 neces- 

 sary compensation the reduction of some adjoining part. 



