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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 longitudinal belts. 1861 1866
which I am fully convinced simultaneously affected the whole world, or at least great meridional belts. 1859 1860
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

allied species, 1859 1860 1861 1866
the several species belong- ing to the same genus, 1869
the several species belonging to the same genus, 1872

(generally of the more dominant forms of life), 1859 1860 1861 1866
OMIT 1869 1872

which separate our 1859 1860 1861 1866
in not only separating, but in apparently forming the 1869 1872

sub-genera, genera, and families; 1859 1860 1861 1866
related species within the same areas; 1869 1872

in 1859 1860 1861 1866
linked together in 1869 1872

linked together by affinity, 1859 1860 1861
linked together, 1866
OMIT 1869 1872

region; according to the nature of the 1859 1860 1861
of the regions, or both; according to the nature of the 1866 1869 1872

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 longitudinal 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.
If the difficulties be not insuperable in admitting that in the long course of time
all the
the
individuals of the same species, and likewise of allied species, have proceeded from some one source; then
I think
....
all the grand leading facts of geographical distribution are explicable on the theory of
migration,
migration
(generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, which separate our several zoological and botanical provinces. We can thus understand the
concentration
localisation
of sub-genera, genera, and families; and how it is that under different latitudes, for instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are in so mysterious a
manner,
manner
linked together by affinity, and are likewise linked to the extinct beings which formerly inhabited the same continent. Bearing in mind that the mutual
relations
relation
of organism to organism
are
is
of the highest importance, we can see why two areas having nearly the same physical conditions should often be inhabited by very different forms of life; for according to the length of time which has elapsed since
new colonists
the colonists
new inhabitants
entered one region; according to the nature of the communication