Chap. V.] MORE VARIABLE THAN GENERIC 123 



to vary in those parts of their structure which have varied within a 

 moderately recent period, and which have thus come to differ. Or 

 to state the case in another manner : — the points, in which all the 

 species of a genus resemble each other, and in which they differ 

 from allied genera, are called generic characters ; and these characters 

 may be attributed to inheritance 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 charac- 

 ters have been inherited from before the period when the several 

 species first branched off from their common progenitor, and subse- 

 quently 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 ad- 

 mitted 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 givo in illus- 



