1915-16.] BOTANICAL SOCIETY OF EDINBURGH 17 



less deserving to be called temperate rain forest, but beyond 

 this the monsoon does not extend. South of latitude 28° 

 the Mekong valley is very much drier than the (monsoon) 

 Salween, and even in the gullies supports little monsoon 

 vegetation, so that the two, separated by a high but narrow 

 mountain range, are in strong contrast. Still further east 

 therefore the change, even on the mountains, is pronounced, 

 and the Mekong- Yangtze divide, instead of being, like the 

 Mekong-Salween divide, clothed with luxuriant forest, is 

 covered with thorny scrub below, coniferous forest above, 

 in which the larch, absent from the Mekong-Salween 

 divide, is predominant at high altitudes. Beyond this range 

 again, in the Yangtze valley, also arid, the flora is typically 

 Chinese, probably without a single Burmese species. 



We have then established these facts, namely, that the 

 monsoon carries as far east as the Salween valley,^ of which 

 the flora (and it may be remarked the fauna also) is closely 

 related to that of the Burmese hinterland ; and secondly, 

 that the Chinese flora is found as far west as the Yangtze 

 valley and Mekong-Yangtze divide, so that the two meet 

 hereabouts, but are sharply divided by the Mekong valley 

 and Mekong-Salween divide. 



Now, it being granted that the Himalayan and Chinese 

 floras are closely related, we can only suppose either that 

 they have been or are at present in close touch with one 

 another, or that both are derived from a common source. 



Owing to the east-and-west trend of the main Asiatic 

 axes of uplift, it is difficult to imagine any common source 

 of supply which is not at one or other end of the axis, thus 

 causing the flora to flow from east to west or vice versa, 

 and pass successively from one region to another. The only 

 alternative is to suppose one of the parallel northern ranges, 

 the flora of which was driven southwards by the advance 

 of the ice, as the common source ; in this way only could 

 the Himalayas and Western China have been peopled simul- 

 taneously instead of successively from a single source. This 

 theoiy assumes that the Himalayas, the north-and-south- 

 trending ranges already referred to, and the tangled moun- 

 tains of Western China must once have had practically the 



1 I.e. south of latitude 28°. North of this point local conditions make 

 the valley extremely arid. The transition is abrupt and startling. 



THAXS. BOT. SOC. EDIN. VOL. XXVTI. 2 



