Chap. VI.] OP TRANSITIONAL VARIETIES. 213 



a narrow and intermediate zone. For forms existiag 

 m larger numbers will have a better chance, within any 

 given period, of presenting further favourable varia- 

 tions for natiiral 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 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 mountains or on the plains, improving 

 their breeds more quickly than the small h\)lders 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 num- 

 bers, will come into close contact with each other, with- 

 out 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 pre- 

 sent an inextricable chaos of varying and intermediate 

 links: first, because new varieties are very slowly formed, 

 for variation is a slow process, and natural selection can 



