Thus, whatever means have sufficed for stocking oceanic islands must
have been to some extent effective in transmitting northern forms from
mountain to mountain, across the equator, to the southern hemisphere;
while for this latter form of dispersal there are special facilities, in
the abundance of fresh and unoccupied surfaces always occurring in
mountain regions, owing to avalanches, torrents, mountain-slides, and
rock-falls, thus affording stations on which air-borne seeds may
germinate and find a temporary home till driven out by the inroads of
the indigenous vegetation. These temporary stations may be at much lower
altitudes than the original habitat of the species, if other conditions
are favourable. Alpine plants often descend into the valleys on glacial
moraines, while some arctic species grow equally well on mountain
summits and on the seashore. The distances above referred to between the
loftier mountains may thus be greatly reduced by the occurrence of
suitable conditions at lower altitudes, and the facilities for
transmission by means of aerial currents proportionally increased.
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