Page - 159 - in The Origin of Species

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-