DIFFICULTIES OF THE THEORY 289 



exists in smaller 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 favorable variations for natural selection to seize 

 on, than will the rarer forms which exist in lesser num- 

 bers. 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 supposing 

 three varieties of sheep to be kept, one adapted to an 

 extensive mountainous region; a second to a compara- 

 tively 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 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, 

 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 varieties are very slowly formed, for 



