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enormously 1859 1860 1861
of time, enormously 1866
of time, immensely 1869 1872

our present 1859 1860 1861
are our present 1866 1869
are our existing 1872

even to conjecture how this could have been effected. Yet, as we have reason to believe that some species have retained the same specific
from
form
for very long
periods
periods,
enormously long as measured by years, too much stress ought not to be laid on the occasional wide diffusion of the same species; for during very long periods
of time
of time
there will always
have been
be
a good chance for wide migration by many means. A broken or interrupted range may often be accounted for by the extinction of the species in the intermediate regions. It cannot be denied that we are as yet very ignorant
as to
of
the full extent of the various climatal and geographical changes which have affected the earth during modern periods; and such changes
may
will
often
obviously
have
greatly
greatly
facilitated migration. As an example, I have attempted to show how potent has been the influence of the Glacial period on the distribution
both
both
of the same and of
allied
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
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
the species
species
of the same genus is in some degree lessened.
As
according to
on
the theory of natural selection an interminable number of intermediate forms must have existed, linking together all the species in each group by
graduations
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