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forms, chiefly of plants, have spread widely throughout the world 1859 1860 1861 1866
species have become widely spread 1869 1872

American 1859 1860 1861 1866
belonging to American 1869 1872

given 1859 1860 1861
allowed 1866 1872
allowed many of 1869

general forms to the whole 1859 1860 1861
fresh-water productions to prevail throughout the 1866
species to prevail through large portions of the 1869
forms to prevail throughout large portions of the 1872

some of the 1861
the 1859 1860 1866 1869 1872

blind 1859 1860 1861
character of most of the blind 1866 1869 1872

closely-allied 1860 1861 1866
closely allied or 1859 1869 1872

showing, in accordance with the foregoing view, that at some former period there has been intercommunication or migration between the two regions. 1859 1860 1861
showing, in accordance with the foregoing views, that at some former period there has been intercommunication or migration between the two regions. 1866
and wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and others as mere varieties; these doubtful forms showing us the steps in the progress of modification. 1869 1872

under different physical conditions, 1859 1860 1861 1866
OMIT 1869 1872

allied to it, 1859 1860 1861 1866
OMIT 1869 1872

is of the widest application throughout nature. We see this on every
mountain-summit,
mountain,
in every lake and marsh. For Alpine species, excepting in
as
so
far as the same forms, chiefly of plants, have spread widely throughout the world during the
recent
recent
Glacial epoch, are related to those of the surrounding
lowlands;
lowlands;—
thus we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants,&c.,
plants, &c.,
all
strictly of
strictly
of strictly
American
forms;
forms,
and it is obvious that a mountain, as it became slowly upheaved, would
naturally
naturally
be colonised from the surrounding lowlands. So it is with the inhabitants of lakes and marshes, excepting in so far
that
as
great facility of transport has given the same general forms to the whole world. We see this same principle in some of the blind animals inhabiting the caves of America and of Europe. Other analogous facts could be given.
It
And it
will, I believe, be
universally
universally
found
universally
to be
to be
true, that wherever in two regions, let them be ever so distant, many closely-allied
representative
or representative
species occur, there will likewise be found some identical
species;
species,
showing, in accordance with the foregoing view, that at some former period there has been intercommunication or migration between the two regions. And wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and some as
mere varieties;
varieties;
these doubtful forms showing us the steps in the
progress
process
of modification.
The
This
relation between the power and extent of migration
in a
in certain
of a
species, either at the present
time
time
or at some former
period,
period
under different physical conditions, and the existence at remote points of the world of
allied
closely-allied
other
species,
species
allied to it, is shown in another and more general way. Mr. Gould remarked to me long ago, that in those genera of birds which range over the world,