110 MULTIPLE AND RUDIMENTARY [CHAP. V. 



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 1 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 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. 

 Xow 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 

 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, without 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 necessary compensation the reduction of some adjoining part. 



Multiple, Rudimentary, and Lowly-organised Structures are 

 Variable. 



It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, 

 both with varieties and species, that when any part or organ is 

 repeated many times in the same individual (as the vertebrae in 

 snakes, and the stamens in polyandrous flowers) the number is 

 variable ; whereas the same part or organ, when it occurs in lesser 

 numbers, is constant. The same author as well as some botanists 

 have further remarked that multiple parts are extremely liable to 

 vary in structure. As "vegetative repetition," to use Pro: 

 Owen's expression, is a sign of low organisation, the forego! 

 statements accord with the common opinion of naturalists, t 

 beings which stand low in the scale of nature are more variab 

 than those which are higher. I presume that lowness here means 

 that the several parts of the organisation have been but litt 

 specialised for particular functions ; and as long as the same 

 has to perform diversified work, we can perhaps see why it shou 

 remain variable, that is, why natural selection should not ha 

 preserved or rejected each little deviation of form so carefully 



