76 FOOTNOTES FROM 



rocky summit of a mountain in one continuous patch, 

 many separate individuals being absorbed in one, must 

 date from fabulous periods. I have gathered it in this 

 form on the summit of Schiehallion, on smooth quartz 

 rocks, which exhibit here and there the glassy polish and 

 deep striae or flutings peculiar to glaciated surfaces, as 

 distinct and unchanged by atmospheric disintegration as 

 though the glacier, which had left these unmistakable 

 -traces behind it, had only yesterday passed over them. 

 And if these ice-marks can be accepted as an indication of 

 the age of the lichen the first and sole organic cover- 

 ing of the rock, be it remembered then in all proba- 

 bility it was in existence during the last great changes 

 of the globe which preceded the creation of the human 

 race. I do not press this point, however, for such a 

 method of computation may be objected to on the score 

 of being inapplicable ; but I think that it is at least as 

 reasonable to believe, that some lichens date their origin 

 as far back as the glacial epoch, as to believe, that there 

 are trees now in existence that were contemporaries of 

 the first generations of men. There are numerous de- 

 structive and obstructive causes, fatal to the longevity of 

 trees, which either do not operate at all, or only to a 

 very limited extent in the economy of lichens ; and, in- 

 deed, these dry, sapless, dormant plants appear to me to 

 possess the power of living for ever, without exhibiting 

 any symptoms of decay, unless from accidental or ex- 

 traneous causes. 



In their geographical distribution, lichens to a certain 

 extent obey the same laws to which the higher orders of 

 vegetation are subject, being influenced by temperature, 



