Chap. VI. TRANSITIONAL VARIETIES. 177 



in smaller numbers in a narrow and intermediate zone. 

 For forms existing in larger numbers will always have 

 a better chance, within any given period, of presenting 

 further favourable 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 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 extensive mountainous region ; a second to a com- 

 paratively narrow, hilly tract ; and a third to 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 moun- 

 tains 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 inter- 

 position 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 pre- 

 sent an inextricable chaos of varying and intermediate 

 links : firstly, because new varieties are very slowly 

 formed, for variation is a very slow process, and 

 natural selection can do nothing until favourable varia- 

 tions chance to occur, and until a place in the natural 



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