Chap. V. LAWS OF VAKIATION. 151 



more rarely to them. The rule being so plainly appli- 

 cable in the case of secondary sexual characters, may be 

 due to the great variability of these characters, whether 

 or not displayed in any unusual maimer — 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 ; and 

 I may here add, that I particularly attended to Mr. 

 Waterhouse's remark, whilst investigating this Order, 

 and I am fully convinced that the rule almost invari- 

 ably holds good with cirripedes. I shall, in my future 

 work, give a list of the more remarkable cases ; I will 

 here only briefly give 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 ex- 

 tremely little even in different 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 some- 

 times wholly unlike in shape ; and the amount of varia- 

 tion in the individuals of several of the species is 

 so great, that it is no exaggeration to state that the 

 varieties differ more from each other in the characters 

 of these important valves than do other species of dis- 

 tinct genera. 



As birds within the same country vary in a remark- 

 ably small degree, I have particularly attended to them, 

 and the rule seems to me certainly to hold good in this 

 class. I cannot make out that it applies to plants, and 

 this would seriously have 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 any species, the fair 



