THEVOICEOFTHEDESERT 124 



Mexico there is a rather common tree with a white ex- 

 fohating bark rather hke that of a sycamore and with curi- 

 ously tapering branches responsible for the common name, 

 elephant tree. At one place and one place only has it ever 

 been found in the United States — namely in a canyon at 

 an elevation of about 4000 feet in the Baboquivari Moun- 

 tains west of Tucson. The nearest Mexican specimen is 

 many miles away. Plants do not leap such distances. It can 

 be pretty safely taken for granted that the Arizona speci- 

 mens were once joined by lines of communication to their 

 fellows in Mexico. 



What broke those lines and isolated these few speci- 

 mens? Why did all the rest of this region become unin- 

 habitable to them? In all probability the elephant tree will 

 be as extinct in the United States as the elephant itself be- 

 came so long ago. And it happens that this particular tree 

 draws attention to itself because it has a very famous close 

 relative in the East. To the Indians in Mexico it supplies a 

 gum which they burn as incense. Its Eastern relative sup- 

 plied the ancient peoples with their frankincense. 



The story of how plants and animals were knowingly 

 or unknowingly introduced by man into different parts of 

 the earth is only part, and usually the clearest part,. of a 

 larger story — how the whole flora and fauna of a given 

 region got to be what it is. By their own efforts plants and 

 animals were advancing and retreating, colonizing and 

 abandoning through all human history as well as through 

 milhons of years before there were any human beings to 

 have a history. In many cases no one can do more than 

 offer a more or less convincing guess concerning what 



