XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 371 
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 un¬ 
occupied 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 trans¬ 
mission by means of aerial currents proportionally increased. 1 
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 summits, 
or for that “ continuous current of vegetation ” described by 
Sir Joseph Hooker as having apparently long existed from 
the northern to the southern hemisphere. It may be admitted 
that we can, possibly, account for the greater portion of the 
floras of remote oceanic islands by the agency of birds alone ; 
because, when blown out to sea land-birds must reach some island 
1 For a fuller discussion of this subject, see my Island Life, chap, xxiii. 
