It may be said, however, that these are the extreme variations, and only
occur in one or two individuals, while the great majority exhibit little
or no difference. Other diagrams will show that this is not the case;
but even if it were so, it would be no objection at all, because these
are the extremes among thirty specimens only. We may safely assume that
these thirty specimens, taken by chance, are not, in the case of all
these species, exceptional lots, and therefore we might expect at least
two similarly varying specimens in each additional thirty. But the
number of individuals, even in a very rare species, is probably thirty
thousand or more, and in a common species thirty, or even three hundred,
millions. Even one individual in each thirty, varying to the amount
shown in the diagram, would give at least a million in the total
population of any common bird, and among this million many would vary
much more than the extreme among thirty only. We should thus have a vast
body of individuals varying to a large extent in the length of the wings
and tail, and offering ample material for the modification of these
organs by natural selection.
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