134 ON THE ORIGIN OF SPECIES 



natural selection and another and adjoining part being 

 reduced by this 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 selection is continually trying to 

 economise in every part of the organisation. If under 

 changed conditions of life a structure before useful 

 becomes less useful, any diminution, however slight, in 

 its development, will be seized on by natural selection, 

 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 other 

 instances could be given : namely, that when a cirripede 

 is parasitic within another 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 extra- 

 ordinary manner with the Proteolepas: 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 basis of the prehensile antennae. 

 Now the saving of a large and complex structure, when 

 rendered superfluous by the parasitic habits of the 

 Proteolepas, though effected by slow steps, 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 individual Proteolepas would 

 have a better chance of supporting itself, by less nutri- 

 ment being wasted in developing a structure now 

 become useless. 



Thus, as I believe, natural selection will always 

 succeed in the long run in reducing and saving every 

 part of the organisation, as soon as it is rendered super- 



