COMPENSATION AND ECONOMY OF GRO^VTH 159 



ally accompanied by a diminished comb and a large beard 

 by diminished wattles. With species in a state of nature it 

 can hardly be maintained that the law is of universal appli- 

 cation ; but many good observers, more especially botanists, 

 believe in its truth. I will not, however, here give any in- 

 stances, for I sec hardly any way of distinguishing between 

 the effects, on the one hand, of a part being largely devel- 

 oped through 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 ad- 

 joining 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 every 

 part of the organisation. If under changed conditions of 

 life a structure, before useful, becomes less useful, its dim- 

 inution will be favoured, for it will profit the individual not 

 to have its nutriment wasted in building up an useless struc- 

 ture. 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 

 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 Proteolepas: for the cara- 

 pace in all other cirripedes consists of the three highly im- 

 portant 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 antennc-e. Now the saving of a 

 large and complex structure, when rendered superfluous, 

 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 would have a better chance of sup- 

 porting 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 be- 



