many species,
though not identical, closely allied, require a separate explanation.
The fact likewise of several of the species on the mountains of Tierra
del Fuego (and in a lesser degree on the mountains of Brazil) not
belonging to American forms, but to those of Europe, though so immensely
remote, requires also a separate explanation.

_Cause of the similarity in the floras of some distant mountains._

Now we may with confidence affirm, from the number of the then floating
icebergs and low descent of the glaciers, that within a period so near
that species of shells have remained the same, the whole of Central
Europe and of North America (and perhaps of Eastern Asia) possessed a
very cold climate; and therefore it is probable that the floras of these
districts were the same as the present Arctic one,--as is known to have
been to some degree the case with then existing sea-shells, and those
now living on the Arctic shores. At this period the mountains must have
been covered with ice of which we have evidence in the surfaces polished
and scored by glaciers. What then would be the natural and almost
inevitable effects of the gradual change into the present more temperate
climate{366}? The ice and snow would disappear from the mountains, and
as new plants from the more temperate regions of the south migrated
northward, replacing the Arctic plants, these latter would crawl{367} up
the now uncovered mountains, and likewise be driven northward to the
present Arctic shores. If the Arctic flora of that period was a nearly
uniform one, as the present one is, then we should have the same plants
on these mountain-summits and on the present Arctic shores. On this view
the Arctic flora of that period must have been a widely extended one,
more so than even the present one; but considering how similar the
physical conditions must always be of land bordering on perpetual frost,
this does not appear a great difficulty; and may we not venture to
suppose that the almost infinitely numerous icebergs, charged with
great masses of rocks, soil and _brushwood_{368} and often driven high
up on distant beaches, might have been the means of widely distributing
the seeds of the same species?

{366} In the margin the author has written "(Forbes)." This may
have been inserted at a date later than 1844, or it may refer to a
work by Forbes earlier than his Alpine paper.

{367} See _Origin_, Ed. i. p. 367, vi. p. 517.