THEVOICEOFTHEDESERT 4g 



for the evolutionist and the plant geographer. How did the 

 cosmopolitans spread from, say, Africa to South America? 

 Why haven't those endemics which are found only in one 

 place set forth to colonize others? How did those which 

 exist in several widely separated but restricted areas come 

 to have this "discontinuous distribution"? 



The answers to those questions when either found or 

 guessed at are various. Obviously a plant species or genus 

 can't migrate past barriers which it cannot cross. A plant 

 adapted, for instance, to desert conditions obviously can't 

 get to another desert thousands of miles away across a 

 damp, humid stretch any more than a moisture-loving 

 plant can cross a desert from one moist region to another. 

 If, as happens to be the case, there are very closely re- 

 lated cosmopolitan genera in Africa and in South America, 

 they must have made their way from one continent to 

 another via some land connections either now or once 

 existing. No less obviously, if endemics are "discontinu- 

 ously distributed" in several small, widely separate areas, 

 that presents a different problem. And in some cases at 

 least they probably represent "relicts" — surviving patches 

 of some once large continuous distribution. 



All this is part of a very large and complicated subject 

 into which we need not try to enter very deeply. But once 

 one has turned from astonishment at the saguaro as an 

 individual to contemplate the fact that it is not to be 

 found anywhere except in this one region where it is so 

 very abundant indeed, one cannot help wondering why 

 that should be. Were there ever saguaros anywhere else? 

 Are they "relicts" or did they first develop here and never 

 get any further? 



Such questions cannot be answered fully and positively 



