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.
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: firstly, because new varieties are very
slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable variations chance to occur,
and until a place in the natural polity
of the country can be better filled by some modification of some one or
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