ORIGINATION OF SELF-GENERATING MATTER 619 
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 
