LAWS OF VARIATION 141 



independently created, why should that part of the 

 structure, which differs from the same part in other 

 independently-created species of the same genus, be 

 more variable than those parts which are closely alike 

 in the several species ? I do not see that any explana- 

 tion can be given. But on the view of species being 

 only strongly marked and fixed varieties, we might 

 surely expect to find them still often continuing 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 the species of some other genus, are called generic 

 characters ; and these characters in common I attri- 

 bute to inheritance 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 pro- 

 genitor, 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 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 variable than those parts of 

 the organisation which have for a very long period 

 remained constant. 



In connection 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 



