] 59 ^"^^^ ^"^ *^^ mountain top 



most bare rocks is what happened on others so long ago 

 that that bare rock is now overlaid with pine forest. 



The secret lies, of course, in that thin, dry-looking en- 

 crustation, gray or greenish or yellow, which covers part 

 of the surface of many of even the barest rocks. The 

 casual eye takes it so for granted that it is hardly noticed. 

 And even when noticed it looks less like a living thing 

 than like some sort of sediment on, or exudation from, 

 the rock itself. Yet insignificant as it appears, it is an in- 

 dispensable link. Only because it could grow there where 

 nothing else had ever grown before, is the mountain ul- 

 timately carpeted with plants and clothed with forests. 



No life seems dimmer than the life of these lichens. 

 They grow so slowly that some individuals may just pos- 

 sibly be among the longest-lived organisms on earth. They 

 also grow almost everywhere — on tree trunks and fences 

 as well as upon walls and boulders, from the steaming 

 tropics to the arctic tundra. They may even, so some 

 sober scientists think, grow on the greenish planet Mars 

 where no other organism we know anything about could 

 possibly survive. Almost the only places they do not thrive 

 are smoky industrial cities. 



Everywhere they suggest life at its gentlest, most sub- 

 dued and most enduring. Nothing else except, perhaps, 

 the bacteria seems to ask so little in order to stay alive. 

 And slowly and persistently as they grow they seem to 

 grow to no purpose. They are so humble that even Tho- 

 reau was a little apologetic about his interest in them, 

 and they are so unpromising in appearance that it was not 

 until after Thoreau's day that the secret of their structure 

 and physiology was deciphered. Yet they were here mil- 

 lions of years before we were; they not improbably may 



