LAWS OF VARIATION 217 



tance from a common progenitor, for it can rarely have 

 happened that natural selection will have modified sev- 

 eral 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 char- 

 acters have varied and come to differ since the period 

 when the species branched off from a common progen- 

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

 degree variable — at least more variable than those parts 

 of the organization 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 organization: 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 



— Science — 10 



