Chap. V. LAWS OF VARIATION. 157 



is less rigid in its action than ordinary selection, as it 

 does not entail death, but only gives fewer offspring 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 suc- 

 ceeded 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 lughest 

 importance, because common to large groups ; but in 

 certain genera the neuration differs in the different spe- 

 cies, 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 descended from the same progenitor, 

 as have the two sexes of any one of the species. Con- 

 sequently, whatever part of the structure of the common 

 progenitor, or of its early descendants, became variable ; 

 variations of this part would, it is highly probable, be 

 taken advantage of by natural and sexual selection, in 



