XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 371 



have the unexplored snow mountains of Xew Guinea, the 

 Bellenden Ker mountains in Queensland, and the Ne^v England 

 and Blue ^Mountains of New South Wales. Between Brazil 

 and Bolivia the distances are no gi'eater ; while the u?d)roken 

 range of mountains from Arctic America to Tierra-del-Fuego 

 offers the greatest facilities for transmission, the partial gaj) 

 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 

 gi'ow 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 occuiTence of suitable 

 conditions at lower altitudes, and the facilities for trans- 

 mission by means of aerial currents proportionally increased.^ 



Facts Explained hij 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, 230ssibly, 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 



^ For a fuller discussion of this subject, see luy Island Life, cliap. xxiii. 



