214 ABSENCE OE EAEITY [Chap. VI. 



natural selection can do nothing until favourable in- 

 dividual differences or variations occur, and until a 

 place in the natural polity of the country can be better 

 filled by some modification of some one or more of its 

 inhabitants. And such new places will depend on slow 

 changes of climate, or on the occasional immigration of 

 new inhabitants, and, probably, in a still more important 

 degree, on some of the old inhabitants becoming slowly 

 modified, with the new forms thus produced and the old 

 ones acting and reacting on each other. So that, in 

 any one region and at any one time, we ought to see 

 only a few species presenting slight modifications of 

 structure in some degree permanent ; and this assuredly 

 we do see. 



Secondlv, areas now continuous must often have 

 existed within the recent period as isolated portions, in 

 which many forms, more especially amongst the classes 

 which unite for each birth and wander much, may 

 have separately been rendered sufficiently distinct to 

 rank as representative species. In this case, inter- 

 mediate varieties between the several representative 

 species and their common parent, must formerly have 

 existed within each isolated portion of the land, but 

 these links during the process of natural selection will 

 have been supplanted and exterminated, so that they 

 will no longer be found in a living state. 



Thirdly, when two or more varieties have been formed 

 In different portions of a strictly continuous area, inter- 

 mediate varieties will, it is probable, at first have been 

 formed in the intermediate zones, but they will 

 generally have had a short duration. For these 

 intermediate varieties will, from reasons already as- 

 signed (namely from what we know of the actual 



