, inhabiting distant and isolated regions, as the process of
modification has necessarily been slow, all the means of migration
will have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in
some degree lessened.

As according to the theory of natural selection an interminable number
of intermediate forms must have existed, linking together all the
species in each group by gradations as fine as our existing varieties,
it may be asked, Why do we not see these linking forms all around us?
Why are not all organic beings blended together in an inextricable
chaos? With respect to existing forms, we should remember that we
have no right to expect (excepting in rare cases) to discover DIRECTLY
connecting links between them, but only between each and some extinct
and supplanted form. Even on a wide area, which has during a long period
remained continuous, and of which the climatic and other conditions of
life change insensibly in proceeding from a district occupied by one
species into another district occupied by a closely allied species, we
have no just right to expect often to find intermediate varieties in
the intermediate zones. For we have reason to believe that only a few
species of a genus ever undergo change; the other species becoming
utterly extinct and leaving no modified progeny. Of the species which do
change, only a few within the same country change at the same time;
and all modifications are slowly effected. I have also shown that
the intermediate varieties which probably at first existed in the
intermediate zones, would be liable to be supplanted by the allied forms
on either hand; for the latter, from existing in greater numbers,
would generally be modified and improved at a quicker rate than the
intermediate varieties, which existed in lesser numbers; so that
the intermediate varieties would, in the long run, be supplanted and
exterminated.

On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why
is not every geological formation charged with such links? Why does
not every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? Although geological
research has undoubtedly revealed the former existence of many links,
bringing num