166 ORIGIN OF SPECIES 



since the period when the species branched off from a com- 

 mon progenitor, it is probable that they should still often be 

 in some degree variable, — at least more variable than those 

 parts of the organisation which have for a very long period 

 remained constant. 



Secondary Sexual Characters Variable. — I think it will be 

 admitted by naturalists, without my entering on details, that 

 secondary sexual characters are highly variable. It will also 

 be admitted that species of the same group differ from each 

 other more widely in their secondary sexual 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 dis- 

 played, with the amount of difference between the females. 

 The cause of the original variability of these characters is 

 not manifest; but we can see why they should not have been 

 rendered as constant and uniform as others, for they are 

 accumulated by sexual selection, which is less rigid in its ac- 

 tion than ordinary selection, as it does not entail death, but 

 only gives fewer offspring to the less favoured males. What- 

 ever 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 have suc- 

 ceeded in giving to the species of the same group a greater 

 amount of difference in these than in other respects. 



It is a remarkable fact, that the secondary differences be- 

 tween the two sexes of the same species are generally dis- 

 played in the very same parts of the organisation in which 

 the species of the same genus differ from each other. Of 

 this fact I will give in illustration the two first instances 

 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 common to very large groups of beetles, but 

 in the Engidas, as Westwood has remarked, the number varies 

 greatly; and the number likewise differs in the two sexes of 

 the same species. Again in the fossorial hymenoptera, the 

 neuration of the wings is a character of the highest impor- 

 tance, because common to large groups ; but in certain genera 

 the neuration differs in the different species, and likewise in 



