Their
mutual relations will thus have been in some degree disturbed; consequently
they will have been liable to modification; and they have been modified;
for if we compare the present Alpine plants and animals of the several
great European mountain ranges, one with another, though many of the
species remain identically the same, some exist as varieties, some as
doubtful forms or sub-species and some as distinct yet closely allied
species representing each other on the several ranges.
In the foregoing illustration, I have assumed that at the commencement of
our imaginary Glacial period, the arctic productions were as uniform round
the polar regions as they are at the present day. But it is also necessary
to assume that many sub-arctic and some few temperate forms were the same
round the world, for some of the species which now exist on the lower
mountain slopes and on the plains of North America and Europe are the same;
and it may be asked how I account for this degree of uniformity of the
sub-arctic and temperate forms round the world, at the commencement of the
real Glacial period. At the present day, the sub-arctic and northern
temperate productions of the Old and New Worlds are separated from each
other by the whole Atlantic Ocean and by the northern part of the Pacific.
During the Glacial period, when the inhabitants of the Old and New Worlds
lived further southwards than they do at present, they must have been still
more completely separated from each other by wider spaces of ocean; so that
it may well be asked how the same species could then or previously have
entered the two continents.
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