156 SECONDARY SEXUAL Chap. V. 



which are closely alike in the several species? I do not sec 

 that any explanation can be given. But on the vie'.v that spe- 

 cies are only stron2;ly marked and fixed varieties, Ave might ex- 

 pect to find them still oil en continuing to vary in those parts 

 of their structure which had varied witliin a moderately-recent 

 period, and which had thus come to diCfer, 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 charac- 

 ters in common I attribute to inheritance from a common pro- 

 genitor, for it can rarely have happened that natural selection 

 Avill have modified several species, fitted to more or less wide- 

 ly-diflerent habits, in exactly the same manner : and as these 

 so-called generic characters have been inherited from before 

 tlie period when the different 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 probaljle that tliey should vary at the present day. On the 

 other hand, the points in which species differ from other spe- 

 cies of the same genus are called specific characters ; and as 

 these specific characters have varied and come to differ since 

 tlie period when the species branched off from a common pro- 

 genitor, 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 con- 

 stant. 



Secondary Sexual Characters variable. 



In coimection with the present subject, I Avill 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 diller from each other more widely in their second- 

 ary sexual characters tlian in other parts of their organization; 

 (^ompare, for instance, tlie amount of difference between the 

 males of gallinaceous birds, in which secondary sexual charac- 

 ters are strongly displayed, with the amount of difference be- 

 tween the females; and the tnith of tliis jiroposition will be 

 granted. Tiie cause of the original variability of secondary 

 S(\xual characters is not manifest ; but we can see why these 

 characters should not have been rendered as constant and imi- 

 form as other parts of the organization; for secondary sexual 

 characters have been accumulated by sexual selection, which is 



