SOIL PRODUCED BY SUCCESSIVE TRIBES. 37 



influence, is frequently accompanied by so great a change in the 

 general appearance of the plant, that the same species growing 

 in dark and moist places, in which the fructification was not 

 evolved, has been considered to belong to a distinct kind from 

 the perfect specimen. No true Lichens are ever found in mines, 

 caverns, or other places deprived of light ; nor are there any that 

 grow entirely under water ; although some species, which con- 

 nect this group with the Sea-weeds, grow on the sea-shore, 

 where they are alternately submerged and left dry by the tide. 



37. To the Lichens may well be applied the title of Vernaculi, 

 or bond-slaves, which Linnzeus fancifully gave to the Sea-weeds, 

 regarding them as fettered to the rocks on which they grow. 

 For the Lichens seem as it were chained to the soil, which they 

 labour to improve for the benefit of others, although they derive 

 no nourishment from it themselves. The mode in which they 

 prepare the sterile rock, for the reception of plants that require 

 a higher kind of nourishment, is most remarkable. They may 

 be said to dig for themselves graves, for the reception of their 

 remains, when death and decay would otherwise speedily dissi- 

 pate them. For whilst living, these Lichens form a considerable 

 quantity of oxalic acid, (which is a peculiar compound of car- 

 bon and oxygen, two ingredients supplied by the atmosphere, 

 Chap, vi.) ; and this acts chemically upon the rock, (especially 

 if of limestone,) forming a hollow which retains the particles of 

 the structure, when their term of connected existence has expired. 

 The moisture which is caught in these hollows, finds its way 

 into the cracks and crevices of the rocks ; and, when frozen, 

 rends them into minute fragments by its expansion, and thus 

 adds more and more to the forming soil. Successive generations 

 of these bond-slaves continuously and indefatigably perform their 

 duties ; until at length, as the result of their accumulated toil, 

 the barren and insulated rocks, or the pumice or lava of the 

 volcano, become converted into fruitful fields. For when Flora's 

 standard has once been planted on tracts thus claimed, they are 

 soon colonised by plants of other tribes. The Mosses, Ferns, and 

 other Cryptogamia follow them ; and at last, by the growth and 

 rlecay of successive generations of plants, a sufficient thickness of 



