LAWS OF VARIATION 137 



not displayed in any unusual manner — of which fact 

 I think there can be little doubt. But that our rule ia 

 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 

 distinct 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 

 presumption is that it is of high importance to that 

 species ; nevertheless the part in this case is eminently 

 liable to variation. MTiy 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 

 have descended from other species, and have been 



