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1859
1860
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1869
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1859
1861
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1869
1872

occurred within the same period; 1859 1860 1861 1866
probably occurred,— 1869 1872

transport,— a subject which has hardly ever been properly experimentised on; 1859 1860
transport,— a subject which has hardly ever been properly experimented on; 1861 1866
trans- port,— 1869
transport,— 1872

how 1859 1860 1861 1866 1869
and this is a very important consideration, how 1872

tracts, I think 1859 1860
tracts, 1861 1866
tracts,— 1869 1872

in 1859 1860 1861 1866
is not insuperable in 1869 1872

the same parents, are not insuperable. 1859 1860 1861 1866
common parents. 1869 1872

and 1859 1860 1861 1866
of all kinds, and 1869 1872

change most 1859 1860 1861 1866
have changed very 1869 1872

I do not think that 1859 1860 1861 1866
OMIT 1869 1872

insuperable; though they often are 1859 1860 1861 1866
far from insuperable; though 1869 1872

extremely great. 1860 1861 1866
extremely grave. 1859
they are often great. 1869 1872

has been the influence of the modern 1859 1860 1861 1866
a part the 1869
a part the last 1872

which I am fully convinced simultaneously affected the whole world, or at least great meridional belts. 1859 1860
which I am fully convinced simultaneously affected the whole world, or at least great longitudinal belts. 1861 1866
has played, which affected even the equatorial regions, and which, during the alternations of the cold in the north and south, allowed the productions of opposite hemispheres to mingle, and left some of them stranded in all parts of the world on the mountain-summits. 1869
has played, which affected even the equatorial regions, and which, during the alternations of the cold in the north and south, allowed the productions of opposite hemispheres to mingle, and left some of them stranded on the mountain-summits in all parts of the world. 1872

changes of climate and of the level of the land, which have certainly occurred within the recent period, and of other
similar
similar
changes which
may
may
have occurred within the same period; if we remember how
profoundly
profoundly
ignorant we are with respect to the many
and
and
curious means of occasional transport,— a subject which has hardly ever been properly experimentised on; if we bear in
mind,
mind
how often a species may have ranged continuously over a wide area, and then have become extinct in the intermediate tracts, I think the
difficulty
difficulties
in believing that all the individuals of the same species, wherever
found, are
located, have
descended from the same parents, are not insuperable. And we are led to this conclusion, which has been arrived at by many naturalists under the designation of single centres of creation, by
various
some
general considerations, more especially from the importance of barriers and from the analogical distribution of sub-genera, genera, and families.
With respect to
the
the
distinct species
belonging to
of
the same genus, which on
our
my
theory
must
must
have spread from one parent-source; if we make the same
allowance
allowances
as before for our ignorance, and remember that some forms of life change most slowly, enormous periods of time
having been
being
thus granted for their migration, I do not think that the difficulties are insuperable; though they often are in this case,
as
and
in that of the individuals of the same species, extremely great.
As exemplifying the effects of climatal changes on distribution, I have attempted to show how important has been the influence of the modern Glacial
period
period,
which I am fully convinced simultaneously affected the whole world, or at least great meridional belts. As showing how diversified are the means of occasional transport, I have discussed at some little length the means of dispersal of fresh-water productions.