TRANSITIONAL VARIETIES 183 



quickly than the small holders on the intermediate narrow, 

 hilly tract; and consequently 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, with- 

 out the interposition of the supplanted, intermediate hill 

 varietv. 



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

 defined objects, and do not at any one period present an inex- 

 tricable chaos of varying and intermediate links : first, be- 

 cause new varieties are very slowly formed, for variation is 

 a slow process, and natural selection can do nothing until 

 favourable individual differences or variations occur, and un- 

 til 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 to see only a few species presenting slight modifi- 

 cations of structure in some degree permanent; and this as- 

 suredly 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 ren- 

 dered sufficiently distinct to rank as representative species. 

 In this case, intermediate varieties between the several repre- 

 sentative 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, intermediate 

 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 



