ountains of the warmer regions of the earth and in the southern hemisphere
are of doubtful value, being ranked by some naturalists as specifically
distinct, by others as varieties; but some are certainly identical, and
many, though closely related to northern forms, must be ranked as distinct
species.

Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large {377} body of geological evidence,
that the whole world, or a large part of it, was during the Glacial period
simultaneously much colder than at present. The Glacial period, as measured
by years, must have been very long; and when we remember over what vast
spaces some naturalised plants and animals have spread within a few
centuries, this period will have been ample for any amount of migration. As
the cold came slowly on, all the tropical plants and other productions will
have retreated from both sides towards the equator, followed in the rear by
the temperate productions, and these by the arctic; but with the latter we
are not now concerned. The tropical plants probably suffered much
extinction; how much no one can say; perhaps formerly the tropics supported
as many species as we see at the present day crowded together at the Cape
of Good Hope, and in parts of temperate Australia. As we know that many
tropical plants and animals can withstand a considerable amount of cold,
many might have escaped extermination during a moderate fall of
temperature, more especially by escaping into the lowest, most protected,
and warmest districts. But the great fact to bear in mind is, that all
tropical productions will have suffered to a certain extent. On the other
hand, the temperate productions, after migrating nearer to the equator,
though they will have been placed under somewhat new conditions, will have
suffered less. And it is certain that many temperate plants, if protected
from the inroads of competitors, can withstand a much warmer climate than
their own. Hence, it seems to me possible, bearing in mind that the
tropical productions were in a suffering state and could not have presented
a firm front against intruders, that a certain number of the more vigorous
and dominant temperate forms might have penetrated the native ranks and
have reached or {378} even crossed the equator. The invasion would, of
course, have been greatly favoured by high land, and perhaps by a dry
climate; for Dr. Falconer informs me that it is the