124 SECONDARY SEXUAL [Chap. V. 



tration 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 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 crusta- 

 ceans 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 Ihe 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 several 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 extreme variability of any part which is deve- 

 loped 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 ; — that the great variability of 

 secondary sexual characters, and their great difference in closely 

 allied species ; — that secondary sexual and ordinary specific differ- 

 ences are generally displayed in the same parts of the organisation, 

 — are all principles closely connected together. All being mainly 

 due to the species of the same group being the descendants of 

 a common progenitor, from whom they have inherited much in 

 common, — to parts which have recently and largely varied being 

 more likely still to go on varying than parts which have long been 

 inherited and have not varied — to natural selection having more or 



