cies 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
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 {204} 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 intermedia