128 ABSENCE OR RARITY [CHAP. VL 



number of well-marked varieties that 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 t\to 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 inextric- 

 able 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 favourable individual 

 differences or variations occur, and until a place in the natural 

 pclity 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 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 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 

 already assigned (namely from what we know of the actual distri- 



