112 UNUSUALLY DEVELOPED PARTS [CHAP. 



future work, give a list of all the more remarkable cases ; I will 

 .here give only one, as it illustrates the rule in its largest applica- 

 tion. The opercular valves of sessile cerripedes (rock barnacles) 

 are, in every sense of the word, very important structures, and 

 they differ extremely little even in distinct genera; but in the 

 several species of one genus, Pyrgoma, these valves present a 

 marvellous amount of diversification ; the homologous valves in 

 the different species being sometimes wholly unlike in shape ; and 

 the amount of variation in the individuals of the same species is 

 so great, that it is no exaggeration to state that the varieties of 

 the same species differ more from each other in the characters 

 derived from these important organs, than do the species belong- 

 ing to other distinct genera. 



As with birds the individuals of the same species, inhabiting 

 the same country, vary extremely little, I have particularly 

 attended to them ; and the rule certainly seems to hold good in 

 this class. I cannot make out that it applies to plants, and this 

 would have seriously shaken my belief in its truth, had not the 

 great variability in plants made it particularly difficult to com- 

 pare their relative degrees of variability. 



When we see any part or organ developed in a remarkable 

 degree or manner in a species, the fair presumption is that it is of 

 high importance to that species: nevertheless it is in this case 

 eminently liable to variation. Why should this be so 1 On the 

 view that each species has been independently Created, with all 

 its parts as we now see them, I can see no explanation. But on 

 the view that groups of species are descended from some other 

 species, and have been modified through natural selection, I think 

 we can obtain some light. First let me make some preliminary 

 remarks. If, in our domestic animals, any part or the whole 

 animal be neglected, and no selection be applied, that part (for 

 instance, the comb in the Dorking fowl) or the whole breed will 

 cease to have a uniform character : and the breed may be said to 

 be degenerating. In rudimentary organs, and in those which 

 have been but little specialised for any particular purpose, and 

 perhaps in polymorphic groups, we see a nearly parallel case ; for 

 in such cases natural selection either has not or cannot have come 

 into full play, and thus the organisation is left in a fluctuating 

 condition. But what here more particularly concerns us is, that 

 those points in our domestic animals, which at the present time 

 are undergoing rapid change by continued selection, are also 

 eminently liable to variation. Look at the individuals of the same 

 breed of the pigeon, and see what a prodigious amount of differ- 

 ence there is in the beaks of tumblers, in the beaks and wattle of 

 carriers, in the carriage and tail of fantaiis, tc., these being the 

 points now mainly attended to by English fanciers. Even in the 



