STRUCTURES VARIABLE 161 



A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY 



DEGREE OR MANNER, IN COMPARISON WITH THE 



SAME PART IN ALLIED SPECIES, TENDS TO 



BE HIGHLY VARIABLE 



Several years ago I was much struck by a remark, to the 

 above effect, made by Mr. Waterhouse. Professor Owen, 

 also, seems to have come to a nearly similar conclusion. 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 unusu- 

 ally developed in one species or in a few species in compari- 

 son 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 remarkable man- 

 ner in comparison with the other species of the same genus. 

 The rule applies very strongly in the case of secondary sex- 

 ual 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 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 charac- 

 ters. The rule being so plainly applicable in the case of sec- 

 ondary 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 : I 

 particularly attended to Mr. Waterhouse's remark, whilst 

 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 



F— HC XI 



