186 UNUSUALLY DEVELOPED PARTS. [Chap. V. 



production. The rule applies to males and females; 

 but more rarely to the females, as they seldom oSer 

 remarkable secondary sexual characters. The rule be- 

 ing 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 her- 

 maphrodite cirripedes; 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 applica- 

 tion. The opercular valves of sessile cirripedes (rock 

 barnacles) are, in every sense of the word, very im- 

 portant 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 cer- 

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

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



