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I believe that during the slow process of modification 1859 1860 1861 1866
OMIT 1869 1872

species will have been 1859 1860 1861 1866
same species inhabiting the same area will be 1869 1872

will not have been due, 1859 1860 1861 1866
OMIT 1869 1872

to 1859 1860 1861 1866
will not be due to 1869 1872

facts. 1859 1860 1861 1866 1869
facts. Change of climate must have had a powerful influence on migration. 1872

Change of climate must have had a powerful influence on migration: a region when 1859 1860 1861 1866
Change of climate must have had a powerful influence on migration; an impassable region when 1869
A region now impassable to certain organisms from the nature of 1872

was different may 1859 1860 1861 1866
was different from what it now is, may 1869
might 1872

but now be impassable; 1859 1860 1861 1866
when the climate was different. 1872
OMIT 1869

a single parent. But in the
great majority
majority
of cases, namely, with all organisms which habitually unite for each birth, or which
occasionally
often
intercross, I believe that during the slow process of modification the individuals of the species will have been kept nearly uniform by intercrossing; so that many individuals will
go
have gone
on simultaneously changing, and the whole amount of modification will not have been due, at each
stage
stage,
to descent from a single parent. To illustrate what I mean: our English
race-horses
racehorses
differ
slightly
slightly
from the horses of every other breed; but they do not
own
owe
their
superiority
difference
and
difference
superiority
to descent from any single pair, but to continued care in
the selecting
selecting
and training
of many
many
individuals during
each
many
generation.
generations.
Before discussing the three classes of facts, which I have selected as presenting the greatest amount of difficulty on the theory of
"single
"single
centres of
creation,"
creation,"
I must say a few words on the means of dispersal.
Means
Means
of
of
Dispersal.
Dispersal .
Dispersal .—
Sir C. Lyell and other authors have ably treated this subject. I can give here only the briefest abstract of the more important facts. Change of climate must have had a powerful influence on migration: a region when its
climate,
climate
was different may have been a high road for
migration;
migration,
but now be impassable; I shall, however, presently have to discuss this branch of the subject in some detail. Changes of level in the land must also have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend
together, or
or
may formerly have
blended.
blended:
Where
where
the sea now extends, land may at
a
a
former
periods
period
have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other.