ocess 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 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 at least possible. For instance,
a swim-bladder has apparently been converted into an air-