Aridity and Evolution. 



225 



The development of desert conditions in the manner de- 

 scribed over a region of any extent would entail the least dis- 

 turbance on mountain summits, where by reason of the lowered 

 temperature and the facilities for condensation, the evaporating 

 power of the air would remain lowest. The original or pre-desert 

 forms would be able to maintain themselves on such elevated 

 slopes with but little adjustment. Similar survivals might ensue 

 along the lower drainage lines, where the underflow in stream- 

 ways and washes might support a moisture-loving vegetation as 

 it does in southwestern Africa and southwestern America. 



So much for survival by localization. A second manifesta- 

 tion would consist in a restriction of seasonal activities. The rate 

 of evaporation on the lower levels might be lessened by lower 

 temperatures during the winter season and at this time rapidly 

 acting annual plants with broad leaves and mesophytic habit 

 might develop from sprouting seeds and carry through their 

 cycle of activity, remaining dormant in the form of heavily coated 

 seeds during the warmer, dryer period of the year. Perennials 

 with deciduous leaves might display a coincident activity. 

 This survival of moisture-loving plants in a region of pronounced 

 desert character is most marked, however, in places where the 

 precipitation occurs within definite moist or rainy seasons, such 

 as the great Sonoran desert, in w r hich twx> maxima of precipita- 

 tion occur separated by periods of extreme drought. Both the 

 winter and summer rainy seasons are characterized by the luxur- 

 iant growth of broad-leaved annuals of a mesophytic habit, 

 which might not be distinguished from those of any moist region. 

 Some species are active during the summer season, and others 

 during the winter, while a smaller number perfect seeds during 

 both seasons. A number of perennials parallel this activity of 

 the annuals, with the result that in the most arid parts of Arizona 

 half of the rlative species are in no sense desert plants, requiring 

 as much moisture for their development as do those of Maryland, 

 Michigan or Florida. The desiccation of a region is seen, there- 

 fore, not to result in the extermination of moisture-loving types, 

 but rather in the reduction of their relative importance and a 

 limitation of their activities to brief periods or moist seasons. 



