Chap. VI.] OE TRADITION AT, VARIETIES 73? 



slowly modified and improved. 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 supposing three varieties of sheep to be 

 kept, one adapted to an extensive 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 favour 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 improved hill breed ; and thus the 

 tAvo 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 inextricable 

 chaos of varying and intermediate links : first, because new varie- 

 ties are very slowly formed, for variation is a slow process, and 

 natural selection can do nothing until favourable 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 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 modifications 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 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 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 



