Chap. V.J CHARACTERS VARIABLE. 193 



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 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 ofEer excellent illustra- 

 tions 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 printipally given by these organs." This re- 

 lation 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 sexual selection, in order to fit the sev- 

 eral species to their several places in the economy of 

 nature, and likewise to fit the two sexes of the same 

 species to each other, or to fit the males to struggle 

 with other males for the possession of the females. 



Finally, then, I conclude that the greater variability 

 of specific characters, or those which distinguish species 

 from species, than of generic characters, or those which 

 are possessed by all the species; — ^that the frequent ex- 

 treme variability of any part which is developed in a 

 species in an extraordinary manner in comparison with 

 the same part in its congeners; and the slight degree of 

 variability in a part, however extraordinarily it may be 

 developed, if it be common to a whole group of species; 



