294 D. T. M AC DOUG A L 



within a year, while the agencies most effective in their revegetation 

 are combined wind action and flotation. 



Many areas, such as the central basin of Asia, the American desert, 

 the Eyre Basin in Australia, and southern Africa, offer clear examples 

 of the effect of desiccation upon the vegetation of a region, but when 

 we proceed to the consideration of the probable happenings when 

 an arid region receives an increasing precipitation, our speculations 

 must be based wholly on experimental evidence of the physiological 

 behavior of plants under known conditions. 



Here, as in the decrease of the supply of water, no mass move- 

 ment or extermination of a flora is to be taken for granted. Many 

 highly specialized succulents extremely local in their distribution 

 would undoubtedly quickly perish with the progression of a climate 

 bringing an excess of moisture; alterations in temperature would not 

 exercise such violent action upon plants of wider range, however. 

 That both together might not totally exterminate a type of succulent 

 is shown by the existence of cacti in tropical rain-forests and on the 

 high northern plains of Nevada, Idaho, and Montana. If plants 

 of wider latitudinal distribution are taken into consideration it may 

 be seen that with an advance of polar climate to the southward the 

 extermination of a species in the northern part of its range would be 

 coincident with additions to the eligible area on the southward. If 

 the land area were limited or if mountain barriers intervened, such 

 dissemination would of course be impracticable and the forms 

 involved would soon perish. These features must be taken into 

 account in an interpretation of the flora of the inclosed basin of 

 central Asia, which, so far as the meager information available 

 shows, is extremely poor in the higher succulent desert types, a 

 characteristic also of the Death Valley and of the Salton Basin in 

 North America. 



The unfavorable influence of increasing moisture upon the 

 xerophytic forms of a region would also include effects of an indirect 

 character. Soil temperature and moisture relations would undergo 

 great alterations, humus would increase, and other changes would 

 ensue, entailing conditions which their specialized structures would 

 be unfitted to meet. Furthermore, succulent and spinose plants 

 being advanced types, their retrogressive evolution to conform to 

 t 



