218 THE ORIGIN OF SPECIES 



accumulated by sexual selection, which is less rigid in 

 its action than ordinary selection, as it does not entail 

 death, but only gives fewer offspring to the less favored 

 males. Whatever tlie 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 tliese 

 than in other respects. 



It is a remarkable fact that the secondary differences 

 between the two sexes of the same species are generally 

 displ.iyed in the very same parts of the organization in 

 which the species of the same genus differ from each 

 other. Of this fact I will give in illustration the first 

 two 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 Engidae, as 

 West wood has remarked, the number varies greatly; and 

 the number likewise differs m the two sexes of the same 

 species. Again in the fossorial hymenoptera, the neura- 

 tion 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 the two sexes of the same species. Sir J. 

 Lubbock has recently remarked that several minute crus- 

 taceans offer excellent illustrations of this law. "In 

 Pontella, for instance, the sexual characters are afforded 

 mainly by the anterior antennae and by the fifth pair of 

 legs: the specific differences also are principally given 

 by these organs." This relation has a clear meaning on 



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