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 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 warmest
spots. 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 even crossed the equator. The
283