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 could only glide through the
air.
We have seen that a species may under new conditions of life change
its habits, or have diversified habits, with some habits very unlike
those of its nearest congeners. Hence we can understand, bearing in
mind that each organic being is trying to live wherever it can live,
how it has arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits of auks.
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