LICHENS. 119 
geological character of the rocks upon which they 
are produced ; and thus several species and even 
genera are necessarily rare and confined to par- 
ticular localities. It may, however, be said of 
them in general that they are cosmopolitan, 
universally distributed over the surface of the 
globe, and capable of existing in almost every 
situation, from the calcined plains of Africa to the 
snow-mantled pinnacles of Spitzbergen. Placed 
almost at the lowest scale of organization, they 
often require nothing more for their conservation, 
than the moisture of the atmosphere precipitated 
on naked masses of rock; and their simple form 
and structure enable them to resist an amount 
alike of heat and cold, sufficient to destroy all 
vitality in more perfectly organized plants. In 
the Arctic regions—those outer boundaries of the 
earth, where eternal winter presides—these humble 
plants constitute by far the largest proportion of 
the flora, and by their prodigious development, 
and their wide social distribution, give as marked 
and peculiar a character to the scenery, as the 
palms and tree-ferns impart to the landscapes of 
the tropics. In the southern hemisphere also, 
lichens almost extend to the pole. They mark 
the extreme limit at which land vegetation has 
been found ; one shrubby species, with large, deep, 
chestnut-coloured fructification, called Usnea fas- 
