150 LAWS OF VAEIATION. Chap. V. 



rudimentary parts are left to the free play of the vari- 

 ous laws of growth, to the effects of long-continued dis- 

 use, and to the tendency to reversion. 



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 with a remark, nearly to the 

 above effect, published by Mr. Waterhouse. I infer 

 also from an observation made by Professor Owen, with 

 respect to the length of the arms of the ourang-outang, 

 that he has come to a nearly similar conclusion. It is 

 hopeless to attempt to convince any one of the truth of 

 this 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 comparison with 

 the same part in closely allied species. Thus, the bat's 

 wing is a most abnormal structure in the class mam- 

 malia ; but the rule would not here apply, because there 

 is a whole group of bats having wings ; it would apply 

 only if some one species of bat had its wings developed 

 in some 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, applies to characters 

 which are attached to one sex, but are not directly 

 connected with the act of reproduction. The rule ap- 

 plies to males and females ; but as females more rarely 

 offer remarkable secondary sexual characters, it applies 



