52 TRANSACTIONS OF THE [Sess. lxxx 



the similarit}^ of the Mekong- Sal ween and Salween-Irra- 

 waddy floras also extends to the alpine flora, of course 

 derived from the north. In the valleys we find the same 

 thing. As far north as latitude 28", where these rivers, 

 breaking through from Tibet, flow in narrow arid trenches, 

 cut ofl'from the rain-bearing winds by the western ranges, 

 and still further desiccated by the indraught of hot air 

 rushing through them, we find at least indications of an 

 Indo-Malayan flora which has spread up from the south. 

 In the Salween valley this is obvious enough, as there are 

 palms, giant bamboos, Aspleniurii Nidus, Linn., and other 

 ferns, Aroids, orchids, and other typical Burmese (monsoon) 

 plants ; in the case of the narrow Mekong valley, however, 

 it is only in the shaded gullies that these monsoon plants 

 have a chance of establishing themselves, and there we find 

 Musa, Asclepiadaceae, ferns, Citrus, and other Burmese 

 plants. The flora of the Yangtze valley is much more 

 Chinese. Before the parallel divides had reached any great 

 height, or before the Sino-Himalayan range had been 

 breached, when the Burmese hinterland was a big lake, 

 and the monsoon extended eastwards along the southern 

 slope of the Sino-Himalayan range, all this country would 

 be covered with monsoon forest, and what now remains is 

 evidently the remnant after the advance of the Himalayan 

 and Chinese floras consequent on the rise of the mountains 

 and cutting ofl" of the monsoon rainfall. 



Suinmary. 



I have shown that the distribution of floras on the 

 Mekong-Yangtze and Mekong-Sal ween divides is in accord- 

 ance with the theory that the parallel divides have been 

 pushed up one by one from tlic west, the first to appear 

 being the easternmost ; also that these two divides and 

 the Salween- Irrawaddy divide derived their floras from 

 a common source which was probably in the west, as shown 

 by the number of Himalayan Primula sections found on 

 them. It could not, however, be overlooked that the 

 similarity of flora extended well into China, and for this 

 reason I suggested an old Sino-Himalayan range of which 

 two broken portions now remain, separated by a great gap ; 

 also that the advance of the ice in Western China had driven 



