LAWS OF VARIATION 211 



Owen, also, seems to have come to a nearly similar con- 

 clusion. It is hopeless to attempt to convince any one of 

 the truth of the above proposition without giving the 

 long array of facts which I have collected, and which 

 cannot possibly be here introduced. I can only state my 

 conviction that it is a rule of high generality. I am 

 aware of several causes of error, but I hope that I have 

 made due allowance for them. It should be understood 

 that the rule by no means applies to any part, however 

 unusually developed, unless it be unusually developed in 

 one species or in a few species in comparison with the 

 same part in many closely allied species. Thus, the wing 

 of a bat is a most abnormal 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 re- 

 markable 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 char- 

 acters, used by Hunter, relates to characters which are 

 attached to one sex, but are not directly connected with 

 the act of reproduction. The rule applies to males and 

 females; but more rarely to the females, as they seldom 

 offer remarkable secondary sexual characters. The rule 

 being so plainly applicable in the case of secondary sex- 

 ual 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 

 cirripeds; I particularly attended to Mr. Waterhouse's 



