hemispheres caused by the sun
being in _perihelion_ in the winter of the one while it was in _aphelion_
during the same season in the other, would necessarily lead to increased
aerial and marine currents, as already explained; and whenever geographical
conditions were such as to favour the production of glaciation in any area
these effects would become more powerful, and would further aid in the
dispersal of the seeds of plants.

_Changes of Climate Favourable to Migration._--It is clear then, that
during periods when no glacial epochs were produced in the northern
hemisphere, and even when a mild climate extended over the whole polar
area, alternate changes of climate favouring the dispersal of plants would
occur on all high mountains, and with particular force on such as rise
above the snow-line. But during that long-continued, though comparatively
recent, phase of high excentricity which produced an extensive glaciation
in the northern hemisphere and local glaciations in the southern, these
risings and lowerings of the snow-line on all mountain ranges would have
been at a maximum, and {518} would have been increased by the depression of
the ocean which must have arisen from such a vast bulk of water being
locked up in land-ice, and which depression would have produced the same
effect as a general elevation of all the continents. At this time, too,
aerial currents would have attained their maximum of force in both
hemispheres; and this would greatly facilitate the dispersal of all
wind-borne seeds as well as of those carried in the plumage or in the
stomachs of birds, since we have seen, by the cases of the Azores and
Bermuda, how vastly the migratory powers of birds are increased by a stormy
atmosphere.

_Migration from North to South has been long going on._--Now, if each phase
of colder and warmer mountain-climate--each alternate depression and
elevation of the snow-line, only helped on the migration of a few species
some stages of the long route from the north to the south temperate
regions, yet, during the long course of the Tertiary period there might
well have arisen that representation of the northern flora in the southern
hemisphere which is now so conspicuous. For it is very important to remark
that it is not the existing flora alone that is represented, such as might
have been conveyed during the last glacial epoch only; but we find a whole
series of northern types evidently of varying degrees of antiquity