240 THE ORIGIN OF SPECIES 



variation is a slow process, and natural selection can do 

 notliing until favoral)le individual differences or variations 

 occur, and until a place in the natural polity of the coun- 

 try 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 pro- 

 duced 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. 



Secondly, areas now continuous must often have ex- 

 isted within the recent period as isolated portions, in 

 which many forms, more especially among 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 varieties 

 between the several representative species and their com- 

 mon parent, must formerly have existed within each 

 isolated portion of the land, but these lini^s 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 intermedi- 

 ate varieties will, from reasons already assigned (namely 



