Chap. V.J STRUCTURES VARIABLE. 119 



why natural selection should not have preserved or rejected each 

 little deviation of form so carefully as when the part has to serve 

 for some one special purpose. In the same way that a knife which 

 has to cut all sorts of things may he of almost any shape ; whilst 

 a tool for some particular purpose must be of some particular 

 shape. Natural selection, it should never he forgotten, can act 

 solely through and for the advantage of each being. 



Eudimentary parts, as it is generally admitted, are apt to be 

 highly variable. We shall have to recur to this subject ; and I will 

 here only add that their variability seems to result from their use- 

 lessness, 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. Watcrhouse. 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 

 Avhich 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 

 sexual characters, may be due to the great variability of these charac- 

 ters, whether or not displayed in any unusual manner — of which fact 1 

 think there can be little doubt. But that our rule is not confined 

 to secondary sexual characters is clearly shown in the casa of 



