1915-16.] BOTANICAL SOCIETY OF EDINBURGH 23 



filled with a terminal moraine, above which fragments of 

 a glacier still lingered. 



Having satisfied ourselves that the ice is actually retreat- 

 ing from the Mekong- Yangtze and Mekong-Salween divides, 

 we must ask another question : — Is this due to an actual 

 diminution of the monsoon rainfall, or simply to a local 

 deflection or cutting off" of the rain-bearing winds ? 



Now the direction of the monsoon, blowing alternately 

 from the S.W. in summer and the N.E. in winter, is 

 primarily dependent on the rotation of the earth ; and the 

 actual existence of the monsoon, its intensity, and the 

 amount of moisture it carries, on the main distribution of 

 the ocean and continental land masses;^ and since it is 

 almost certain that no appreciable change has taken place 

 in an}^ of these factors within times so geologically recent 

 as those during which the events we are recording took 

 place — say, within Tertiary times — it follows that any 

 marked decrease in the monsoon rainfall must be ascribed 

 to local causes, namely, a deflection or cutting off" of the 

 rain-bearing winds. It might, of course, be objected that 

 the retreat of the ice was due in the first instance to a 

 general rise of temperature over the whole region, and not 

 to diminished precipitation at all. But the fact that the 

 glaciers on the Mekong-Salween divide have been affected 

 considerably less than those on the Mekong- Yangtze divide 

 while those on the Salween-Irrawaddy divide have probably 

 been still less afi"ected — even if they have retreated at all, 

 which may be doubted — points to another cause. If there 

 has been a general rise of temperature, wh}' should it aflfect 

 the glaciers on one range more than those on another ? 



Tlie Remnant Flora. 



I have said that the retreat of the glaciers is due to a 

 diminution of rainfall, and thereby tacitly assumed that 

 the monsoon, or something very like it, was once felt 

 further east. In that case the Mekong- Yangtze and 

 Mekong-Salween divides must once have had very similar 

 floras, whereas it has been pointed out already that their 



1 The relative distribution of land and sea along the continental 

 shelf has, of course, changed appreciably within Tertiary times, but not 

 their relative proportions, nor their distribution in bulk. 



