llied, forming an
exactly parallel case to the species found on the various mountain
summits which have been referred to. The distances from Madagascar to
the South African mountains and to Kilimandjaro, and from the latter to
Abyssinia, are no greater than from Spain to the Azores, while there are
other equatorial mountains forming stepping-stones at about an equal
distance to the Cameroons. Between Java and the Himalayas we have the
lofty mountains of Sumatra and of North-western Burma, forming steps at
about the same distance apart; while between Kini Balu and the
Australian Alps we have the unexplored snow mountains of New Guinea,
the Bellenden Ker mountains in Queensland, and the New England and Blue
Mountains of New South Wales. Between Brazil and Bolivia the distances
are no greater; while the unbroken range of mountains from Arctic
America to Tierra-del-Fuego offers the greatest facilities for
transmission, the partial gap between the lofty peak of Chiriqui and the
high Andes of New Grenada being far less than from Spain to the Azores.
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.[181]

_Facts Explained by the Wind-Carriage of Seeds._

But if we altogether reject aerial transmission of seeds for great
distances, except by the agency of birds, it will be difficult, if not
impossible, to account for the presence of so many identical species of
plants on remote mountain