150 COMPENSATION OF GROWTU. Chap. V. 



species in a state of nature it can hardly be maintained that 

 the law is of universal application ; but many jrood observers, 

 more especially botanists, believe in its truth. I will not, how- 

 ever, here give any instances, for I see hardly any way of dis- 

 tinguishing between the effects, on the one hand, of a part 

 being largel}' developed through natural selection and another 

 and adjoining part being reduced by this same process or by 

 disuse, and, on the other hand, the actual willidrawal of nutri- 

 ment from one part owing to the excess of growth in another 

 and adjoining part. 



I suspect, also, that some cases of compensation which have 

 been advanced, and likewise some other facts, may be merged 

 inider a more general principle, namel}', that natural selection 

 is continually trying to economize in every part of the organ- 

 ization. If under changed conditions of life a structure before 

 useful becomes less useful, any diminution, however slight, in 

 its development, Avill be seized on by natural selection, for it 

 will ]iroiit 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 cirri- 

 pedes, 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 extraordinary manner with the Proteolepas ; for the car- 

 apace in all other cirripedes 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 

 i.s reduced to the merest rudiment attached to the bases of the 

 prehensile antenna?. 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 nutriment being wasted in develop- 

 ing 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 organi- 

 zation, as soon as it is rendered by changed habits of life super- 

 fluous, without by any means causing some other part to be 

 largely develojicd in a corresponding degree; and, converse- 



