Chap. V.] STRUCTURES VARIABLE. 185 



variability seems to result from their uselessness, and 

 consequently 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 he 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 

 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 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 connected with the act of 



