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permitted their former 1859 1860 1861
formerly permitted their 1866 1869 1872

chiefly from the distribution of the fossil Gnathodon), 1859 1860
OMIT 1861

during their long southern migration and re-migration northward, will 1859 1860 1861
moved first southward and afterwards backwards to the north, in unison with the changing climate, they will not 1866 1869 1872

to nearly the same climate, 1859 1860 1861
during their long migrations to any great diversity of temperature, 1866
during their long migrations to any great diversity of temperature; 1869 1872

is especially to be noticed, they will have kept 1859 1860 1861
they will all have migrated 1866
they all migrated 1869 1872

and those of the Pyrenees, as remarked by Ramond, are more especially allied to the plants of northern Scandinavia; those of the United States to Labrador; those of the mountains of Siberia to the arctic regions of that country. These views, grounded as they are on the perfectly well-ascertained occurrence of a former Glacial period, seem to me to explain in so satisfactory a manner the present distribution of the Alpine and Arctic productions of Europe and America, that when in other regions we find the same species on distant mountain-summits, we may almost
conclude,
conclude
without other evidence, that a colder climate permitted their former migration across the
low
low
intervening
lowlands, now
tracts, since
become too warm for their existence.
If the climate, since the Glacial period, has ever been in any degree warmer than at present (as some geologists in the United States believe to have been the
case),
case,
chiefly from the distribution of the fossil Gnathodon), then the arctic and temperate productions will at a very late period have marched a little
farther
further
north, and subsequently have retreated to their present homes; but I have met with no satisfactory evidence with respect to this intercalated slightly warmer period, since the Glacial period.
As the
The
arctic
forms
forms,
during their long southern migration and re-migration northward, will have been exposed to nearly the same climate,
and
and,
as is especially to be noticed, they will have kept in a body
together,
together;
consequently
consequently
their mutual relations will not have been much
disturbed.
disturbed,
Hence,
and,
in accordance with the principles inculcated in this volume,
these forms
they
will not have been liable to much modification. But with
the
our
Alpine productions, left isolated from the moment of the returning warmth, first at the bases and ultimately on the summits of the mountains, the case will have been somewhat
dif-
dif-