the United States, every plant is identical with species growing in
Labrador. In these cases all ordinary means of transport fail. Most of
the plants have heavy seeds, which could not possibly be carried such
immense distances by the wind; and the agency of birds in so effectually
stocking these Alpine heights is equally out of the question. The
difficulty was so great, that some naturalists were driven to believe
that these species were all separately created twice over on these
distant peaks. The determination of a recent glacial epoch, however,
soon offered a much more satisfactory solution, and one that is now
universally accepted by men of science. At this period, when the
mountains of Wales were full of glaciers, and the mountainous parts
of Central Europe, and much of America north of the great lakes, were
covered with snow and ice, and had a climate resembling that of Labrador
and Greenland at the present day, an Arctic flora covered all these
regions. As this epoch of cold passed away, and the snowy mantle of the
country, with the glaciers that descended from every mountain summit,
receded up their slopes and towards the north pole, the plants receded
also, always clinging as now to the margins of the perpetual snow line.
Thus it is that the same species are now found on the summits of the
mountains of temperate Europe and America, and in the barren north-polar
regions.
But there is another set of facts, which help us on another step towards
the case of the Javanese mountain flora. On the higher slopes of
the Himalayas, on the tops of the mountains of Central India and of
Abyssinia, a number of plants occur which, though not identical with
those of European mountains, belong to the same genera, and are said by
botanists to represent them; and most of these could not exist in the
warm intervening plains. Mr. Darwin believes that this class of facts
can be explained in the same way; for, during the greatest severity of
the glacial epoch, temperate forms of plants will have extended to the
confines of the tropics, and on its departure, will have retreated up
these southern mountains, as well as northward to the plains and hills
of Europe. But in this case, the time elapsed, and the great change of
conditions, have allowed many of these plants to become so modified that
we now consider them to be distinct species. A variety of other facts
of a similar nature have led him to believe that the depression of
temperature was at one time sufficient to allow a few north-temperate
plants to cross the Equator (by the most elevated routes) and to reach
the Antarctic regions, where they are now found. The evidence on which
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