14 Permanence and Evolution. 



parts which never vary, that variation has 

 its limits. But this is not at all the whole 

 of what is probably meant. In the first 

 place, we must not consider only or mainly 

 the points of the topography of the organic 

 system in which differences may happen to be 

 located, but also much more the nature of the 

 differences themselves, just as in some diseases 

 it is only the unscientific who classify them 

 according to the parts of the organism which 

 they may attack, e.g., all eye diseases, all affec- 

 tions of the chest, etc. Secondly, such dis- 

 tinctions are important as go along with and 

 serve as an index to pervading differences 

 running through the whole system, e.g., differ- 

 ences hi the dental formula ; those on the other 

 hand are unimportant which exist between 

 forms in all other respects identical or nearly 

 so, e.g., differences in colour. Granting, then, 

 that natural selection with spontaneous variation 

 could within the period of history develop out 



