156 LAWS OF VAKIATION. Chap. V. 



progenitor, for it can rarely have happened that natural 

 selection will have modified several 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 a remote period, since that period 

 when the 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 cha- 

 racters ; and as these specific characters have varied 

 and come to differ within the period of the branching 

 off of the species from a common progenitor, it is pro- 

 bable 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. 



In connexion with the present subject, I will make 

 only two other remarks. I think it will be admitted, 

 without my entering on details, that secondary sexual 

 characters are very variable ; I think it also will be 

 admitted that species of the same group differ from 

 each other more widely in their secondary sexual cha- 

 racters, than in other parts of their organisation ; com- 

 pare, 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 their females ; and the truth of this 

 proposition will be granted. The cause of the original 

 variability of secondary sexual characters is not mani- 

 fest ; but we can see why these characters should not 

 have been rendered as constant and uniform as other 

 parts of the organisation ; for secondary sexual charac- 

 ters have been accumulated by sexual selection, which 



