Chap. VI.] OF TEAXSITIONAL VAKIETIES. 213 



numbers in a narrow and intermediate zone. For forms 

 existing in larger numbers will have a better chance, 

 within any given period, of presenting further favour- 

 able variations for natural selection to seize on, than 

 will the rarer forms which exist in lesser numbers. 

 Hence, the more common forms, in the race for life, will 

 tend to beat and supplant the less common forms, for 

 these will be more 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 moun- 

 tainous 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 selec- 

 tion ; 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 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 tolerably 

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

 present an inextricable chaos of varying and inter- 

 mediate links : first, because new varieties are very 

 slowly formed, for variation is a slow process, and 



