Chap, v.] STRUCTURES VARIABLE. 185 



bility seems to result from their uselessness, and conse- 

 quently from natural selection having had no power to 

 check deviations in their structure. 



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 un- 

 usually 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 ab- 

 normal 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 spe- 

 cies 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 man- 

 ner. 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 re- 



