TIBET AND PAMIRS-TSAIDAM 75 



and some have floors of loose and dry pastures of tuft- 

 grass. Meagre crops of barley and vegetables are raised 

 almost up to the foot of the glaciers on their alluvial 

 fans ; terraces are resorted to for the purpose of irrigation. 

 These valleys are well sheltered, and perhaps unusually 

 mild, considering the altitude ; yet because they receive 

 only a very scanty rainfall and depend exclusively on 

 their ice-fed rivers, vegetation is limited to the floors, the 

 more so still on account of the steepness of the slopes. 



Tsaidam, in the north-east corner of Tibet, is inter- 

 mediate in elevation between it and the Gobi. It is 

 a secluded and desolate waste of salt swamps and loose 

 brushes of tuft-grass, intermixed with lifeless deserts, 

 but, though treeless, it does not completely exclude 

 scrub vegetation : a few shrubs, such as tamarisks, 

 charmiks (lycium), with their edible berries, nitrarias 

 and buckthorns sometimes twenty feet high, ascend to 

 altitudes of 9,000 and 10,000 feet. 



Being absorbed entirely in the bitter struggle against 

 nature, the Tibetans, who have remained primarily 

 shepherds, have received civilizing influences from the 

 surrounding countries without contributing anything of 

 importance to the progress of mankind. 



CHAPTEE II 



NORTH AMERICA 



Two large l bodies of land like North America and 

 Eurasia, placed in similar situations and possessing 

 broadly similar relations to air and sea currents, are 

 bound to show a great similarity in the main features 

 of the distribution of climate, and of life : an increase 



