214 ABSENCE OB EARITY [Chap. VL 



do nothing until favourable individual 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, prob- 

 ably, 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 struc- 

 ture in some degree permanent; and this assuredly we 

 do see. 



Secondly, 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, intermedi- 

 ate 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 gen- 

 erally have had a short duration. For these inter- 

 mediate varieties will, from reasons already assigned 

 (namely from what we know of the actual distribution 



