| → 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 |
|
|
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
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
with very little identity, between the productions of North America and Europe,— a relationship which is
remarkable, considering the distance of the two areas, and their separation by the
Ocean. We can further understand the singular fact remarked on by several
that the productions of Europe and America during the
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
of their inhabitants. |
|
| During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which
the New and Old Worlds,
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.
the plants and animals migrated southward, they
→become
mingled in the one great region with the native American productions, and
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
|
