fluence
of the Glacial period on the distribution both of the same and of
representative species throughout the world. We are as yet profoundly
ignorant of the many occasional means of transport. With respect to
distinct species of the same genus inhabiting very 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 species
of the same genus is in some degree lessened.

As on 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 present 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 climate and other conditions of life change
insensibly in going 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 zone.
For we have reason to believe that only a few species are undergoing
change at any one period; and all changes are slowly effected. I have
also shown that the intermediate varieties which will at first probably
exist in the intermediate zones, will be liable to be supplanted by the
allied forms on either hand; and the latter, from existing in greater
numbers, will generally be modified and improved at a quicker rate than
the intermediate varieties, which exist in lesser numbers; so that
the intermediate varieties will, 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? We meet with no such
evidence, and this i