THE VOICE OF THE DESERT 128 



of plants which fell over the line perished. Only man is 

 limitlessly mobile. Only he creates everywhere the environ- 

 ment which he must have. The jay and the squirrel hide 

 acorns and nuts for the winter. But they have never 

 learned the trick of establishing food bases in inhospitable 

 territory, much less how to travel with, as Thoreau puts 

 it, "shiploads of preserved meats" and, at jom-ney's end, 

 with the empty cans "piled sky-high for a sign." 



Any mountain range in southern Arizona will provide 

 the opportunity for a journey in altitude and any will tell 

 two stories. The first is this life-zone story of what is; the 

 second, almost as clear, is the story of what was. All the 

 mountains were once not only higher than they are, but 

 also more barren — originally, indeed, completely so. Even 

 now the soil is usually thin and great rocky peaks jut 

 through it. Every plant and every animal that lives there 

 descends from a race of colonists and each of the succes- 

 sive waves of colonization prepared the way for the one 

 which came after. There could be no squirrels until there 

 were oaks or pines to supply them with food. There could 

 be no oaks or pines until generations of smaller plants 

 had grown and died to make the soil in which they grow. 

 But these smaller plants could not have grown upon the 

 bare rocks when they were first lifted high in the air some 

 hundreds of millions of years ago. How did it all start? 

 By what steps did life conquer the forbidding lifeless 

 steeps? 



Before I leave the heights I usually like to look again 

 at the answer to this question. And the process is still 

 going on. Many bare rocks are still completely bare unless 

 one looks closely. And what is happening on these al- 



