DIFFICULTIES ON THEORY 159 



then, I think, we can understand why intermediate 

 varieties should not endure for very long periods ; — 

 why as a general rule they should be exterminated and 

 disappear, sooner than the forms which they originally 

 linked together. 



For any form existing in lesser numbers would, as 

 already remarked, run a greater chance of being exter- 

 minated 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 a far more important 

 consideration, as I believe, is that, during the process 

 of further modification, by which two varieties are 

 supposed on my theory to be converted and perfected 

 into two distinct species, the two which exist in larger 

 numbers from inhabiting larger areas, will have a great 

 advantage over the intermediate variety, which exists 

 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 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 

 to be kept, one adapted to an extensive mountainous 

 region ; a second to a comparatively 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 chance? 

 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 



