192 SECONDARY SEXUAL [Chap. V. 



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 fe- 

 males. 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 action than ordinary selection, 

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

 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 

 have succeeded 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 

 between the two sexes of the same species are generally 

 displayed 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 cLaraeter com- 

 mon to very large groups of beetles, but in the 

 Engidffi, as Westwood has remarked, the number varies 

 greatly; and the number likewise differs in the two 



