ii4 Deserts 



if, while alive, the temperature of their sap were 

 affected by climate, or by the changes of summer and 

 winter, day and night, then not only would it be con- 

 stantly frozen in the far north, and not far short of 

 boiling in the tropics ; but the sap of an acacia of the 

 desert might freeze by night and almost boil by day 

 a sudden and violent change, which, as has been 

 shown, wears out the very rocks. 



But to return to the ' deserts/ by which we are to 

 understand those regions where water is scarce, 

 drought frequent, and where vegetation, though seldom 

 or never entirely absent, is more or less scanty, and 

 more or less peculiar, because it is especially adapted 

 to the special circumstances of its situation. 



The soil of the desert may, or may not, be poor, but 

 it is the want of water which renders these regions 

 comparatively barren. 



Well-watered, the Kalahari desert might, it is said, 

 be one of the richest grazing lands in the world ; and 

 the utter barrenness of certain tracts of the Sahara is 

 owing merely to the lack of rain, for the soil beneath 

 the sand is actually rich, and is not only quite capable 

 of supporting vegetable life, but is extremely fertile 

 wherever there is moisture. 



The other marked characteristic of desert-lands is 

 the dearth, if not absence, of trees, and the question 

 we have now to consider is whether these two charac- 

 teristics the want of water and the scarcity of all 

 vegetation, but especially of trees are brought about 

 the one by the other. 



Vegetation cannot thrive, though it may manage to 

 exist, without a regular supply of water ; but does 

 vegetation bring rain or increase the rainfall ? 



