160 ON THE ORIGIN OF SPECIES 



the improved mountain or plain breed will soon take 

 the place of the less improved hill breed ; and thus 

 the two breeds, which originally existed in greater 

 numbers, will come into close contact with each other, 

 without the interposition of the supplanted, inter- 

 mediate hill-variety. 



To sum up, I believe that species come to be toler- 

 ably well-defined objects, and do not at any one period 

 present an inextricable chaos of varying and inter- 

 mediate links : firstly, because new varieties are very 

 slowly formed, for variation is a very slow process, 

 and natural selection can do nothing until favourable 

 variations chance to 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 inhabit- 

 ants. 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 only to see a few species presenting 

 slight modifications of structure in some degree per- 

 manent ; 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, inter- 

 mediate varieties 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 process 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, intermediate varieties will, it is probable, at first 



