he plains; they will, also, have been subsequently
exposed to somewhat different climatical influences. 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. The explanation, I believe, lies in the nature of the
climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world were
specifically the same as now, and we have good reason to believe that
the climate was warmer than at the present day. Hence, we may suppose
that the organisms which now live under latitude 60 degrees, lived
during the Pliocene period further north, under the Polar Circle, in
latitude 66-67 degrees; and that the present arctic productions then
lived on the broken land still nearer to the pole. Now, if we look at
a terrestrial globe, we see under the Polar Circle that there is almost
continuous land from western Europe through Siberia, to