CHAP. V.] MORE VARIABLE THAN GENERIC. 115 



from a common progenitor, for it can rarely have happened that 

 natural selection will have modified several distinct species, fitted 

 to more or less widely-different habits, in exactly the same 

 manner: and as these so-called generic characters have been 

 inherited from before the period when the several species first 

 branched off from their common progenitor, and subsequently 

 have not varied or come to differ in any degree, or only in a slight 

 degree, it is not probable that they should vary at the present 

 day. On the other hand, the points in which species differ from 

 other species of the same genus are called specific characters ; 

 and as these specific characters have varied and come to differ 

 since the period when the species branched off from a common 

 progenitor, it is probable that they should still often be in some 

 degree variable, at least more variable than those parts of the 

 organisation which have for a very long period remained constant. 



Secondary Sexual Characters Variable. I think it will be 

 admitted by naturalists, without my entering on details, that 

 secondary sexual characters are highly variable. 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 organisation : 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 favoured 

 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 

 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 organisation 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 Engidse, 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 import- 



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