140 THE ORIGIN OF SPECIES 



favorable variations for natural selection to seize on, than will the 

 rarer forms which exist in lesser numbers. Hence, the more com- 

 mon forms, in the race for life, will tend to beat and supplant the 

 less common forms, for these will be more slowly modified and im- 

 proved. It is the same principle which, as I believe, accounts for the 

 common species in each country, as shown in the second chapter, 

 presenting on an average a greater number of well-marked varieties 

 than do the rarer species. I may illustrate what I mean by sup- 

 posing three varieties of sheep to be kept, one adapted to an exten- 

 sive mountainous region ; a second to a comparatively narrow, hilly 

 tract; and a third to the wide plains at the base; and that the 

 inhabitants are all trying with equal steadiness and skill to improve 

 their stocks by selection ; the chances in this case will be strongly 

 in favor of the great holders on the mountains or on the plains, 

 improving their breeds more 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 im- 

 proved 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, intermediate hill 

 variety. 



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

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

 ble chaos of varying and intermediate links: first, because new 

 varieties are very slowly formed, for variation is a slow process, 

 and natural selection can do nothing until favorable 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 immigra- 

 tion 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 one acting and re- 

 acting on each other. So that, in any one region and at any one 

 time, we ought to see only a few species presenting slight modifica- 

 tions of structure 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 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 common 



