ically the same; but they are much oftener
specifically distinct, though related to each other in a most remarkable
manner.

This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, Professor Dana, that "it is certainly a
wonderful fact that New Zealand should have a closer resemblance in its
crustacea to Great Britain, its antipode, than to any other part of
the world." Sir J. Richardson, also, speaks of the reappearance on the
shores of New Zealand, Tasmania, etc., of northern forms of fish. Dr.
Hooker informs me that twenty-five species of Algae are common to New
Zealand and to Europe, but have not been found in the intermediate
tropical seas.

It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges
of the intertropical regions, are not arctic, but belong to the northern
temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
from polar towards equatorial latitudes, the Alpine or mountain floras
really become less and less arctic." Many of the forms living on
the mountains 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 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