178 DIFFICULTIES ON THEOKY. Chap. VI. 



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 inha- 

 bitants, and, probably, in a still more important degree, 

 on some of the old inhabitants becoming slowly modi- 

 fied, 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 only to 

 see 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 in 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, intermediate vari- 

 eties between the several representative species and 

 their common parent, must formerly have existed in 

 each broken portion of the land, but these links will 

 have been supplanted and exterminated during the pro- 

 cess of natural selection, so that they will no longer 

 exist 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 gene- 

 rally have had a short duration. For these inter- 

 mediate varieties will, from reasons already assigned 

 (namely from what we know of the actual distribution 

 of closely allied or representative species, and likewise 

 of acknowledged varieties), exist in the intermediate 

 zones in lesser numbers than the varieties which they 

 tend to connect. From this cause alone the interme- 



