80 The Plant World. 



to account for their geological and stratigraphical features. 

 Other well known geologists maintain the same view, and the 

 botanists have made such choice of evidence as was possible under 

 the circumstances, choosing, for the most part, the alternative 

 that permitted the view that at all events the plants of the arid 

 Southwest may have occupied their present habitats during a 

 period much longer than that measured by human occupation. 



But this comfortable assumption appears to be growing un- 

 tenable in the light of recent geographical studies conducted in 

 the vicinity of Tucson, Arizona, by Dr. Ellsworth Huntington, 

 of Yale University, whose well known investigations of desert 

 conditions and changes in various parts of the world entitle his 

 opinionSjto much respect. His contention is that the Hohokam, 

 or prehistoric "unknown people," lived in such numbers in the 

 Santa Cruz Valley and elsewhere in Southern Arizona as to pre- 

 clude, at that time, all possibility of such arid conditions as now 

 prevail there. If his view^ is correct, then the sahuaro, the creo- 

 sote bush, and other desert plants must at that relatively recent 

 time have lived where they do now under widely different con- 

 ditions from those now prevailing, or they must have migrated 

 from elsewhere at a still more recent date. Neither supposition 

 is free from difficulty, and farther data are greatly to be desired. 



