natural selection will always be very slow, and will act, at any one
time, only on a very few forms; and partly because the very process of
natural selection almost 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 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.
Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one;
yet in the case of any organ, if we know of a long series of gradations
in complexity, each good for its possessor, then, under changing
conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural
selection. In the cases in which we know of no intermediate or
transitional states, we should be very cautious in concluding that none
could have existed, for the homologies of many organs and their
intermediate states show that wonderful metamorphoses in function are
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