150 
LAWS OF VARIATION. 
Chap. Y. 
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 he 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 
