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 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 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 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 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.
Pages:
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222