LICHENS. 115 
feet, must be very aged, judging by this standard. 
The foliaceous and shrubby species are the most 
fugacious, though even these have great powers 
of longevity. We have no data from which to 
ascertain the age of tartareous species, which 
adhere almost inseparably to stones. Some of 
them are probably as old as any living organisms 
that exist on the earth. The geographical lichen, 
which often spreads over the whole rocky summit of 
a mountain in one continuous patch, many sepa- 
rate individuals being absorbed in one, must date 
from very remote 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 striz or flutings peculiar 
to glaciated surfaces, as distinct and unchanged 
by atmospheric disintegration as though the gla- 
cier, 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 covering of the rock, be it remem- 
bered—then in all probability it was in existence 
during the last great changes of the globe which 
preceded the introduction of the human race. I 
do not press this point, however, for such a 
method of computation may be objected to; 
but I think that it is at least as reasonable to 
