LAWS OF VARIATION 125 



important organ or part, which is generally very constant through- 

 out a large group of species, differs considerably in closely allied 

 species, it is often variable in the individuals of the same species. 

 And this fact shows that a character, which is generally of generic 

 value, when it sinks in value and becomes only of specific value, 

 often becomes variable, though its physiological importance may 

 remain the same. Something of the same kind applies to mon- 

 strosities: at least Is. Geoff roy Saint-Hilaire apparently entertains 

 no doubt, that the more an organ normally differs in the different 

 species of the same group, the more subject it is to anomalies in 

 the individuals. 



On the ordinary view of each species having been independently 

 created, why should that part of the structure, 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 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 those 

 so-called generic characters have been inherited from before the 

 period when the several species first branched off from their com- 

 mon 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 variable — at least more vari- 

 able 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. 



