■] 27 settlers— old and new 



studies is parallel to the increasing tendency of students 

 in another field to shift their interest from the study of in- 

 dividual men to the study of "society." 



Something was said about all this when we raised the 

 question how and when the cactus got to the Sonoran 

 Desert. But the same question might be asked about any 

 plant or animal anywhere — including, especially, every 

 one that is fitted to live in arid country. When Tribulus 

 terrestris was brought in and found that in many places 

 things were quite to his liking, a member of the same 

 mostly tropical and subtropical family — namely the creo- 

 sote bush — was actually already the dominant shrub in 

 regions drier than Tribulus really likes. How did it get 

 here and why didn't Tribulus come with it? 



We cannot do much more than guess, but probably the 

 immediate ancestor of the creosote crossed some bridge 

 which ceased to exist before the immediate ancestors of 

 Tribulus had developed on the family tree. But the bar- 

 riers once bridged, later impassable, do not by any means 

 have to be oceans. One desert can, for instance, be isolated 

 from another by an intervening barrier of moist fertility 

 just as eflFectively as one moist area may be isolated from 

 another by an intervening desert. When Tribulus was car- 

 ried by man across the barriers it settled down happily to 

 increase and multiply. 



When the question why all animals and plants don't 

 grow everywhere was first asked in scientific rather than 

 mythological terms, the first answer was no doubt that 

 some demand heat and that some won't tolerate it. Proba- 

 bly most men today would give an answer almost as sim- 

 ple. And of course it is true as far as it goes. But it is a 



