156 
LAWS OF VARIATIOK. 
Chap. Y. 
heritance from a common 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 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 within 
the period of the branching off of the species from a 
common progenitor, it is probable that they should still 
often be in some degree variable,—at least more vari¬ 
able 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 
