Chap. V.] MOKE VAKIABLE THAN GENEEIC. 191 



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 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 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 allied genera, 

 are called generic characters ; and these characters may 

 be attributed to inheritance from a common progenitor, 

 for it can rarely have happened that natural selection 

 will have modified several 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 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 variab]e, 

 — at least more variable than those parts of the 

 organisation 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 

 10 



