CHAP. V.] STRUCTURES VARIABLE. Ill 



when the part has to serve for some one special purpose. In the 

 same >vay that a knife which has to cut all sorts of things may be 

 of almost any shape; whilst a tool for some particular purpose 

 must be of some particular shape. Natural selection, it should 

 never be forgotten, can act solely through and for the advantage 

 of each being. 



Rudimentary 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 

 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 t/te 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 convic- 

 tion 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 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 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. Water- 

 house's remark, whilst investigating this Order, and I am fully 

 tonvinced that the rule almost always holds good. I shall, in a 



