162 ORIGIN OF SPECIES 



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 homolo- 

 gous valves in the different species being sometimes wholly 

 Mnlike in shape; and the amount of variation in the indi- 

 viduals of the same species is so great, that it is no exag- 

 geration 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 belonging to other 

 distinct genera. 



As with birds the individuals of the same species, inhabit- 

 ing the same coimtry, vary extremely little, I have particu- 

 larly 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 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 de- 

 scended 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 degen- 

 erating. 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 



