Now, if we may trust these facts and inferences, and
conclude that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then 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 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
it is a far more important consideration, that during the process of
further modification, by which two varieties are supposed 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 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.
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