We have
seen that species at any one period are not indefinitely variable, and are
not linked together by a multitude of intermediate gradations, partly
because the process of natural selection is always very slow, and at any
one time acts only on a few forms; and partly because the very process of
natural selection implies the continual supplanting and extinction of
preceding and intermediate gradations. Closely allied species, now living
on a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be
formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two
forms which it connects; consequently the two latter, during the course of
further modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that
a bat, for instance, could not have been formed by natural selection from
an animal which at first only glided through the air.
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