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much rarer numerically than the forms which they
Now, if we may trust these facts and inferences, and
conclude that varieties linking two other varieties together
existed in lesser numbers than the forms which they connect,
we can understand why intermediate varieties should not endure for very long
as a general
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
forms existing on both sides of it. But
→a
far more important consideration,
→as I believe, is
during the process of further modification, by which two varieties are supposed
→OMIT
to be converted and perfected into two distinct species, the two which exist in larger
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. 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
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