116 FIRST FORMS OF VEGETATION. 
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 contempo- 
raries of the first generations of men. There are 
numerous destructive 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, indeed, 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 extraneous causes. 
We find in the shape and substance of the 
lichens an explanation of their stability and 
permanence. They are specially constructed both 
as regards form and composition for maintain- 
ing a low kind of vitality for indefinite periods, 
and for enduring in the midst of the most 
unfavourable circumstances. As a rule they are 
more or less spherical, and this is the form of 
greatest security, because it possesses the pro- 
perty of greatest symmetry or of equality in the 
relation of many points to one point and to one 
another. When growing they do not break out 
jinto straight lines which represent points separat- 
ing from the general control and actuated by a 
single force in one direction, and are therefore 
unstable and insecure; but they spread by ripples 
