134 m USUALLY DEVELOPED PARTS 



structure in the class of mammals, but the rule would not 

 apply here, because the whole group of bats possesses 

 wings ; it would apply only if some one species had wings 

 developed in a remarkable manner in comparison with the 

 other species of the same genus. The rule applies very 

 strongly in the case of secondary sexual characters, when 

 displayed in any unusual manner. The term, secondary 

 sexual characters, used by Hunter, relates to characters 

 which are attached to one sex, but are not directly con- 

 nected with the act of reproduction. The rule applies to 

 males and females ; but more rarely t< '"he females, as they 

 seldom offer remarkable secondary sexual characte. . 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 charac- 

 ters is clearly shown in the case of hermaphrodite cirri- 

 pedes; I particularly attended to Mr. Waterhouse's remark, 

 while 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 application. The opercular valves of sessile cirri- 

 pedes (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, in- 

 habiting 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 seriously shaken my 

 belief in its truth, had not the great variability in plants 

 made it particularly difficult to compare their relative de- 

 grees of variability. 



When we see any part or organ developed in a remark' 



