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must have been 1859 1860 1861
would have been 1866
would be 1869
will have been 1872

took 1859 1860 1861 1866
must have taken 1869 1872

become 1859 1860 1861 1869 1872
been liable to become 1866

the two Worlds. 1859 1860 1861
Europe and N. America. 1866 1869 1872

New Worlds, began slowly to migrate southwards as the climate became less warm, long before the
com-
....
mencement
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
relationship,
with very little identity, between the productions of North America and Europe,— a relationship which is
highly
most
remarkable, considering the distance of the two areas, and their separation by the
whole Atlantic
Atlantic
Ocean. We can further understand the singular fact remarked on by several
observers
observers,
that the productions of Europe and America during the
latter
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
inter-migration
intermigration
of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which
inha- bited
inhabited
the New and Old Worlds,
had migrated
migrated
south of the Polar Circle, they must have been completely cut off from each other. This separation, as far as the more temperate productions are concerned, took place long ages ago.
As
And as
the plants and animals migrated southward, they
would have
would
will have
become mingled in the one great region with the native American productions, and
would have
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 the two Worlds. Hence it has come, that when we compare