146 8EC0NDABT SEXUAL 0HARACTEE8 VARIABLE. 



pendently created, why should that part of the struc- 

 ture, 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 explanation can be 

 given. But on the view that species are only strongly 

 marked and fixed varieties, we might expect often to find 

 them still continuing to vary in those parts of their strirc- 

 ture 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 char- 

 acters may be attributed to inheritance from a common 

 progenitor, for it can rarely have happened that natural selec- 

 tion will have modified several distinct species, fitted to 

 more or less widely different habits, in exactly the same 

 manner: and as those so-called generic characters have been 

 inherited from before the period when the several species 

 first branched off from their common progenitor, and sub- 

 sequently 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 organization which have for a very long period 

 remained constant. 



SECON-DART SEXUAL CHAEACTEES VAEIABLE. 



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 differ- 

 ence between the males of gallinaceous birds, in which 

 secondary sexual characters are strongly displayed, with 



