] 1 8 Multiple and Rudimentary Chap. V: 



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 

 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 Proteo- 

 lepas : for the carapace in all other cirripedes consists of the three 

 highly-important anterior segments of the head enormously deve- 

 loped, 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 antennas. 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 supporting itself, by less nutriment being wasted. 



Thus, as 1 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 com- 

 pensation 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 vertebras 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 Prof. Owen's 

 expression, is a sign of low organisation, the foregoing statements 

 accord with the common opinion of naturalists, that beings which 

 stand low in the scale of nature are more variable than those which 

 are higher. I presume that lowness here means that the several 

 parts of the organisation have been but little specialised for particular 

 functions ; and as long as the same part has to perform diversified 

 work, we can perhaps see why it should remain variable, that is,, 



