82 FOOTNOTES FROM 



The Lecidea geographica affords, I may mention, the 

 most remarkable example of the almost universal diffu- 

 sion of lichens, being the most Arctic, Antarctic, and 

 Alpine lichen in the world facing the savage cliffs of 

 Melville Island in the extreme north, clinging to the 

 volcanic rocks of Deception Island in the extreme south, 

 and scaling the towering peak of Kinchin-junga, the 

 most elevated spot on the surface of the earth. A 

 catholic beauty, it is to be found in every zone of 

 altitude and latitude " a pilgrim bold in Nature's 

 care." 



On the British mountains we find lichens in great 

 abundance and luxuriance, in spots which favour their 

 growth 'by the humidity continually precipitated from 

 the atmosphere. Most of the species found sparingly 

 scattered at the highest elevations, are identical with 

 those found in the greatest profusion covering immense 

 areas on the plains of Lapland, and on the level of the 

 sea-shore in the Arctic regions ; the isotherms or lines of 

 equal temperature passing through these points. Similar 

 species are also found all over the world below the level 

 of perpetual snow, which on the Alps is 7000 feet, and 

 on the Andes and Himalayas about 15,000 feet. It is 

 somewhat remarkable that Alpine lichens generally are 

 more or less of a brown or black colour. This peculiarity 

 seems to be owing to the presence of usnine or usnic 

 acid, which in a pure state is of a green colour, as in the 

 lichens which grow in shady forests, but which becomes 

 oxidized, and changes to every shade of brown and black, 

 when exposed to the powerful agencies of light and heat, 

 on the bleak barren rocks on the mountain side and 



