228 



The Plant World. 



be included. Some of the forms of these groups now 



inhabit desert areas, although others of similar structure 

 with regard to foliage demand the greatest possible supply of 

 moisture, so that the ascription of any causal relation between 

 the leaf of a pine or cycad and desiccation must be made with 

 many reservations. 



The recession of large expanses of water included in a desic- 

 cating region, such as has occurred in the great basins in Nevada, 

 and in the bolsons to the southward and eastward in New Mexico, 

 Chihuahua and Arizona would present special conditions. The 

 rate at which the waters of such inland seas might recede, how- 

 ever, would be such that the advance of vegetation to cover the 

 emersed areas would be quite as rapid as that necessary to follow 

 a receding ice-sheet or a change of climate due to any cause. 

 Thus our observations on the Salton show that beaches a mile 

 in width may be bared within a year, while the agencies most 

 concerned in their revegetation are combined wind action and 

 flotation by the waters of the lake, together with the action of 

 the small but torrential streams which occasionally rush down 

 the shallow washes carrying the heavier seeds and rocks with 

 equal ease. 



PROBABLE EFFECTS OF INCREASED HUMIDITY. 



Many regions, inclusive of the great central basin of Asia, 

 the deserts of north and south Africa, southern Australia, and 

 of the Americas, with a total area equal to one-sixth of the land 

 surface of the world, offer the most diversified evidence as to 

 the physiographic and vegetational effects of dessication, which 

 have been described, but when the attempt is made to consider 

 the probable happenings consequent upon a reversal of the 

 climatic swing, in which an arid region receives an increasing 

 precipitation, conclusions must be drawn chiefly from experi- 

 mental evidence derived from the laboratory. 



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

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

 Many highly specialized succulents, extremely local in their dis- 

 tribution would undoubtedly quickly perish with the progression 

 of a climate bringing an excess of moisture; alterations in tem- 

 perature 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 



