s, it must be yet more easy for them to traverse continuous areas of
land, whereever mountain-chains offer suitable stations at moderate {513}
intervals on which they might temporarily establish themselves. The
facilities afforded for the transmission of plants by mountains has hardly
received sufficient attention. The numerous land-slips, the fresh surfaces
of broken rock and precipice, the _debris_ of torrents, and the moraines
deposited by glaciers, afford numerous unoccupied stations on which
wind-borne seeds have a good chance of germinating. It is a well-known fact
that fresh surfaces of soil or rock, such as are presented by railway
cuttings and embankments, often produce plants strange to the locality,
which survive for a few years, and then disappear as the normal vegetation
gains strength and permanence.[139] But such a surface {514} will, in the
meantime, have acted as a fresh centre of dispersal; and thus a plant might
pass on step by step, by means of stations temporarily occupied, till it
reached a district {515} where, the general conditions being more
favourable, it was able to establish itself as a permanent member of the
flora. Such, generally speaking, was probably the process by which the
Scandinavian flora has made its way to the southern hemisphere; but it
could hardly have done so to any important extent without the aid of those
powerful causes explained in our eighth chapter--causes which acted as a
constantly recurrent motive-power to produce that "continuous current of
vegetation" from north to south across the whole width of the tropics
referred to by Sir Joseph Hooker. Those causes were, the repeated changes
{516} of climate which, during all geological time, appear to have occurred
in both hemispheres, culminating at rare intervals in glacial epochs, and
which have been shown to depend upon changes of excentricity of the earth's
orbit and the occurrence of summer or winter in _aphelion_, in conjunction
with the slower and more irregular changes of geographical conditions;
these combined causes acting chiefly through the agency of heat-bearing
oceanic currents, and of snow- and ice-collecting highlands. Let us now
briefly consider how such changes would act in favouring the dispersal of
plants.

_Elevation and Depression of the Snow Line as Aiding the Migration of
Plants._--We have endeavoured to show (in an earlier portion of this
volume) that wherever geographical or physical conditions were