1915-16.] BOTANICAL SOCIETY OF EDINBURGH 29 



the glaciers of the former divide retreated, the flora was 

 able to occupy new territory, and, in the inevitable struggle 

 and changed conditions, readapt itself, with the result that 

 new varieties have arisen. But if continuous and free inter- 

 change of seeds from range to range took place — and it may 

 be doubted if, under the most favourable conditions, direct 

 communication could be established for wind-borne seeds 

 except in the alpine region — there is no reason why these 

 alpines should not now be found on both or all three divides. 

 There is one more significant argument — the alpine and 

 sub-alpine floras of the Mekong-Salween and Salween- 

 Irrawaddy divides are more alike than are the same belts 

 on the Mekong-Salween and Mekong- Yangtze divides, and 

 the same is, I think, true in an even greater degree in the 

 case of the respective forest belts. This follows naturally 

 from the fact that the ice has retreated furthest on the 

 Mekonof-Yano^tze divide, little or not at all on the Salween- 

 Irrawaddy divide. Such differences as exist between the 

 floras of the Mekong-Salween and Salween-Irrawaddy 

 divides, in the sub-alpine and forest belts, arise from the 

 greater proportion of monsoon species met with on the 

 latter, a subject which will be referred to again. The 

 Mekong valley is as impassable a barrier south of 

 Ka'-gur-pw as it is to the north, but not so the Salween 

 valley which, as already stated, has a more or less monsoon 

 climate south of latitude 28°, so that direct communication 

 between the Salween-Irrawaddy and Salween-Mekong 

 divides is here not improbable. Some of the plants common 

 to the latter divide and to the Burmese hinterland may 

 have crossed directly from one divide to the other ; but as it 

 is almost certain that the alpine flora common to both 

 divides has travelled down them from the north-west, so is 

 it likely that the southern or monsoon flora, confined chiefly 

 to the forest belt, has travelled up both divides from a 

 common source, and not straight across from the west. 

 The geological history of the western country gives us 

 good grounds for believing all the flora common to the 

 Himalayas, the parallel divides, and the monsoon country 

 to have travelled round the perimeter of a circle, and never 

 across it — a matter which will be referred to presently. 

 Let us now briefly consider the geology of the country. 



