98 



POPULAR HISTORY OP LICHENS. 



quainted with the varieties producible in species by differ- 

 ences in the nature of the habitat, and by climatal and 

 other changes, so shall we probably find the types of liche- 

 nose vegetation in different parts of the world more nearly 

 similar. There is good reason to believe that, hitherto, 

 many v varieties of common species growing on foreign shores 

 have been described by their discoverers as new species, or 

 perhaps as the bases of new genera, from an ignorance of 

 the epirrheology of Lichens, — of the causes productive of 

 changes in form, colour, and general characters. In regard 

 to Polar species, for instance, the peculiarities of climate, 

 the protracted winter, the extreme dryness of the air, and 

 the intensity of light, must be borne in mind ; and it is but 

 reasonable to suppose that British Lichens transferred to the 

 climate of Ross's Islet or Kerguelen's Land, would undergo 

 remarkable modifications in their characters, and in parti- 

 cular would probably become deformed, stunted, rigid, and 

 sterile. The same climatal changes are in operation as we 

 ascend high mountains, viz. the gradual diminution of tem- 

 perature or increase of cold, diminution of moisture or in- 

 crease of dryness of the atmosphere, and the increased in- 

 tensity of the light. We have already seen that Lichens 

 attain their maximum development in tropical regions ; there 



