142 ON THE ORIGIN OF SPECIES 



characters, than in other parts of their organisation ; 

 compare, for instance, the amount of difference between 

 the males of gallinaceous birds, in which secondary 

 sexual characters are strongly displayed, with the 

 amount of difference between their females ; and the 

 truth of this proposition will be granted. The cause 

 of the original variability of secondary sexual characters 

 is not manifest ; but we can see why these characters 

 should not have been rendered as constant and uniform 

 as other parts of the organisation ; for secondary sexual 

 characters have been accumulated by sexual selection, 

 which is less rigid in its action than ordinary selection, 

 as it does not entail death, but only gives fewer off- 

 spring to the less favoured males. Whatever the cause 

 may be of the variability of secondary sexual characters, 

 as they are highly variable, sexual selection will have 

 had a wide scope for action, and may thus readily 

 have succeeded in giving to the species of the same 

 group a greater amount of difference in their sexual 

 characters, than in other parts of their structure. 



It is a remarkable fact, that the secondary sexual 

 differences between the two sexes of the same species 

 are generally displayed in the very same parts of the 

 organisation in which the different species of the same 

 genus differ from each other. Of this fact I will give 

 in illustration two instances, the first which happen to 

 stand on my list ; and as the differences in these cases 

 are of a very unusual nature, the relation can hardly 

 be accidental. The same number of joints in the tarsi 

 is a character generally common to very large groups of 

 beetles, but in the Engidse, as Westwood has remarked, 

 the number varies greatly ; and the number likewise 

 differs in the two sexes of the same species : again in 

 fossorial hymenoptera, the manner of neuration of the 

 wings is a character of the highest importance, because 

 common to large groups ; but in certain genera the 

 neuration differs in the different species, and likewise 

 in the two sexes of the same species. This relation has 

 a clear meaning on my view of the subject : I look at 

 all the species of the same genus as having as certainly 



