Chap. V. 
LAWS OF VAEIATION. 
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 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; 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 
