CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.
4. DISPERSAL DURING THE GLACIAL PERIOD. (continued)
Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view, and
to infer that during some earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly to migrate southwards
as the climate became less warm, long before the commencement of the
Glacial period. We now see, as I believe, their descendants, mostly in a
modified condition, in the central parts of Europe and the United States.
On this view we can understand the relationship with very little identity,
between the productions of North America and Europe--a relationship which
is highly remarkable, considering the distance of the two areas, and their
separation by the whole Atlantic Ocean. We can further understand the
singular fact remarked on by several observers that the productions of
Europe and America during the later tertiary stages were more closely
related to each other than they are at the present time; for during these
warmer periods the northern parts of the Old and New Worlds will have been
almost continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they will have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, must have taken place long ages ago. As the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and would have had to compete with
them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification--for
far more modification than with the Alpine productions, left isolated,
within a much more recent period, on the several mountain ranges and on the
arctic lands of Europe and North America. Hence, it has come, that when we
compare the now living productions of the temperate regions of the New and
Old Worlds, we find very few identical species (though Asa Gray has lately
shown that more plants are identical than was formerly supposed), but we
find in every great class many forms, which some naturalists rank as
geographical races, and others as distinct species; and a host of closely
allied or representative forms which are ranked by all naturalists as
specifically distinct.
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