126 THE ORIGIN OF SPECIES 



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 organization: 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 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 action than ordinary selection, as it does 

 not entail death, but only gives fewer offspring to the less favored 

 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 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 between 

 the two sexes of the same species are generally displayed 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 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 Engidae, 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 importance, 

 because common to large groups ; but in certain genera the neura- 

 tion differs in the different species, and likewise in the two sexes 

 of the same species. Sir J. Lubbock has recently remarked, that 

 several minute crustaceans 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 my view: I look at all the 

 species of the same genus as having as certainly descended from a 

 common progenitor, as have the two sexes of any one species. 

 Consequently, whatever part of the structure of the common 

 progenitor, or of its early descendants, became variable, varia- 

 tions of this part would, it is highly probable, be taken advantage 

 of by natural and sexual selection, in order to fit the several 

 places in the economy of nature, and likewise to fit the two sexes 



