SECONDARY SEXUAL CHARACTERS VARIABLE. 147 



the amount of difference between the females. The cause 

 of the original variability of these characters is not mani- 

 fest; but we can see why they should not have been ren- 

 dered as constant and uniform as others, for they are ac- 

 cumulated 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 selec- 

 tion 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 organization in 

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

 Of this fact I will give in illustration the two first in- 

 stances which happen to stand on my list; and as the dif- 

 ferences 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 Engidse, as West- 

 wood 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 neura- 

 tion of the wings is a character of the highest importance, 

 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 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, variations of this part would, 

 it is highly probable, be taken advantage of by natural and 



