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 
fmdher 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 
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 im¬ 
prove 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 holders on the 
intermediate narrow, hilly tract; and consequently 
the improved mountain or plain breed will soon take 
the place of the less improved hiU 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,,interniediate 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- 
I 3 
