136 ON THE ORIGIN OF SPECIES 



having no power to check deviations in their structure. 

 Thus rudimentary parts are left to the free play of the 

 various laws of growth, to the effects of long-continued 

 disuse, 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 under- 

 stood 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 mammalia ; 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 applies to males 

 and females ; but as females more rarely offer remark- 

 able secondary sexual characters, it applies more 

 rarely to them. 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 



