1915-16.] BOTANICAL SOCIETY OF EDINBURGH 25 



also a species of Fyrola, another lucky find. Less strik- 

 ing examples were Meconopsis pseudo-integrifolia, Prain, 

 Primula pseud o-sikkimensis, G. Forrest, and one or two 

 others which are found scattered on the Mekong- Yangtze 

 divide in favourable localities, but grow in meadows-full on 

 the Mekong-Salween divide associated with plants such as 

 Fritillaria Soidiei, Franch., Aconitum Souliei, Franch., 

 found nowhere on the Mekono-- Yangtze divide. These 

 accidentals, as it were, I have called the remnant flora, 

 as it seems plain they are survivals from a moister climate 

 which have struggled on in a few localities after the bulk 

 of them had perished under new conditions. What these 

 new conditions were I have already indicated — a gradual 

 desiccation owing to the apparent retreat of the monsoon 

 westwards — and both lines of argument (namely, the 

 graduated diminution of precipitation, as indicated by 

 the progressive retreat of the glaciers from the Salweeii- 

 Irrawaddy to the Mekong- Yangtze divide, and the remnant 

 flora of the last-named divide) point to the same cause. 

 We can only suppose therefore that rain screens have been 

 interposed one after the other between the monsoon in the 

 south-west and the dry regions^ east of the Mekong- Yangtze 

 divide — in other words, that these parallel north-and-south- 

 trending ranges have been successively pushed up from the 

 west ; that the rise of the Mekong-Salween divide curtailed 

 the rainfall, and hence impoverished the flora, of the Mekong- 

 Yangtze divide, just as the rise of the Salween-Irrawadd}^ 

 divide is gradually cutting olf tlie rainfall of the Mekong- 

 Salween divide. Each range acts as a rain screen to the 

 next range east of it. Also it is evident that north of 

 Ka'-gur-pw the Mekong-Salween divide has suflered from 

 lack of rain for exactly the same reason as has the Mekong- 

 Yangtze divide further south, namely, the continued inter- 

 polation and elevation of rain screens to the west. It is 

 much less ditflcult to establish the fact of identity between 

 the floras of the Mekong-Salween and Salween-Irrawaddy 

 divides than between the Mekong-Salween and Mekong- 

 Yangtze divides. As already pointed out, desiccation has 

 not proceeded so far in the former case — the Mekong- 



' Baber, Jolm;,tone, Wilson, ami otlier.- have drawn attenliun lo Ihe 

 previous extension of the Szeehwan glaciers. 



