PHYSICAL DEVELOPMENT OF CENTRAL ASIA 639 



When this inner continental area ceased to send its waters to the ocean 

 it was predestined to a course of evolution whose progress must inevita- 

 bly culminate in the desert-waste conditions ruling there today. 



Each of the geological periods mentioned had its characteristic land 

 and water organic life, among which were prophetic ancestral forms in 

 the genealogy of the mammals of today. 



The cause of this differentiating evolution is as simple as it is fate- 

 fully majestic in its progress. The moisture, carried by the high cur- 

 rents of air in their course from the equator to the pole, is largely 

 condensed in rising over the great altitudes of lofty mountain ranges. 

 To the north of the highlands the plains receive but a slight annual 

 precipitation, and this is so distributed in the seasons as to produce the 

 minimum of vegetation in respect to the amount of precipitation received 

 during the year. 



Under these conditions a forest growth is impossible and the surface 

 must be more or less grass-covered or bare, according to the amount of 

 effective precipitation, which in turn may perhaps have varied during 

 different periods with a possible varying in height of the intercepting 

 mountains. 



Under such conditions the region would vary in character between 

 semi-arid and arid. 



Whether semi-arid or arid, the hot air, rising from plains barren of 

 vegetation and heated by the sun of spring and summer, prevents local 

 rainfall, and the residuum of moisture that escaped condensation on the 

 mountains is carried on to the colder regions of the north. It is only 

 during the winter that this residuum is precipitated on the plains as 

 snow, and even this melts away by March, awakening to life a varied 

 desert flora, which in turn vanishes under the burning April sun. 



Thus, excepting the relatively ineffective winter snows, the whole of 

 this vast inner continental region receives waters only from the precipi- 

 tation over the high mountains that separate it from the peripheral zone, 

 and from such mountains as rise sufficiently high within its own area. 



Central Asia from the western border of Manchuria to the western 

 end of the Black sea is a series of great and small land-locked basins. 

 From these no water flows to the ocean, excepting that which the Black 

 sea loses through the canyon of the Bosphorus, which was not opened to 

 the Mediterranean until the present geological epoch. 



This great land-locked area is divided into two basin systems: one is 

 the higher-lying Gobi on the east, inclosed on the west between the moun- 

 tain masses of the Kwenlun and Tienshan. 



