120 Unusually developed Parts highly Variable. Chap. v. 



hermaphrodite cirripedes; I particularly attended to Mr. Water- 

 house's remark, whilst investigating this Order, and I am fully con- 

 vinced that the rule almost always holds good. I shall, in a 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 application. The 

 opercular valves of sessile cirripedes (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 varia- 

 tion 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 impor- 

 tant organs, than do the species belonging 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 vari- 

 ability in plants made it particularly difficult to compare 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 ? 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 selec- 

 tion 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 chance by 

 continued selection, are also eminently liable to variation. Look at 



