212 THE ORIGIN OF SPECIES 



remark, while investigating this Order, and I am fully 

 convinced that the rule almost always holds good. I 

 shall, in a future work, give a list of all the more re- 

 markable cases; I will here give only one, as it illus- 

 trates the rule in its largest application. The opercular 

 valves of sessile cirripeds (rock barnacles) are, in every 

 setise 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 pre- 

 sent a marvellous amount of diversification; the homol- 

 ogous 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 difler more from each other in the charac- 

 ters derived from these important 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 cer- 

 tainly seems to hold good in this class. I cannot make 

 out that it applies to plants, and this would have seri- 

 ously 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 pre- 

 sumption is that it is of high importance to that species: 

 nevertheless it is in this case eminently liable to varia- 

 tion. Why should this be so? On the view that each 

 species has been independently created, with all its parts I 

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



