138 SECONDARY SEXUAL CHARACTERS VARIABLE. ; 



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 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 

 since the period when the species branched off from a com- 

 mon 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. 



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 or- 

 ganization : 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 origi- 

 nal 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 favored 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 giv- 

 ing 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 organization in 

 which the species of the same genus differ from each other. 

 Of this fact I will give in illustration the two first instances 

 which happen to stand on my list : and as the differences in 



