122 THE ORIGIN OF SPECIES 



other species of the same genus. The rule applies very strongly 

 in the case of secondary sexual characters, when displayed in any 

 unusual manner. The term, secondary sexual characters, used by 

 Hunter, relates to characters which are attached to one sex, but 

 are not directly connected with the act of reproduction. The rule 

 applies to males and females; but more rarely to the females, as 

 they seldom offer remarkable secondary sexual characters. The 

 rule being so plainly applicable in the case of secondary sexual 

 characters, may be due to the great variability of these charac- 

 ters, whether or not displayed in any unusual manner — of which 

 fact I think there can be little doubt. But that our rule is not 

 confined to secondary sexual characters is clearly shown in the 

 case of hermaphrodite cirripedes; I particularly attended to Mr. 

 Waterhouse's 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 remarkable cases. I will 

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

 tion. 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 mar- 

 vellous 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 at- 

 tended 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 de- 

 gree 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 obr 



