186 UNUSUALLY DEVELOPED PAETS [Chap. "V. 



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 characters, 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 cirripecles ; I particularly attended to 

 Mr. "Waterhouse's remark, whilst 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 appli- 

 cation. The opercular valves of sessile cirripecles (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 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 



