OF TRANSITIONAL VARIETIES. 153 



For any form existing in lesser numbers would, as already 

 remarked, run a greater chance of being exterminated than 

 one existing in large numbers ; and in this particular case 

 the intermediate form would be eminently liable to the 

 inroads of closely allied forms existing on both sides of it. 

 But it is a far more important consideration, that during tlve 

 process of further modification, by which two varieties are 

 supposed to be converted and perfected into two distinct 

 species, the two which exist in larger numbers, from inhabit- 

 ing larger areas, will have a great advantage over the inter- 

 mediate variety, which exists in smaller numbers in a narjow 

 and intermediate zone. For forms existing in larger num- 

 bers 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 

 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 illus- 

 trate 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 inhabit- 

 ants are all trying with equal steadiness and skill to im- 

 prove their stocks by selection ; the chances in this case 

 will be strongly in favor 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 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 vari- 

 ation is a slow process, and natural selection can do noth- 

 ing until favorable individual differences or variations occur, 

 and until a place in the natural polity of the country can 



