1915-16.] BOTANICAL SOCIETY OF EDINBURGH 41 



direct communication with the Himalayas and is probably 

 only separated from it to-day by the Brahmaputra valley. 

 The eastern divides must soon have severed their connec- 

 tion with the western half of the Sino-Himalayan range 

 (though, as we have seen, the floras of the Mekong- Yangtze 

 and Mekong-Salween divides are practically identical in 

 the north) and were probablj^ never in contact with the 

 eastern half ; for at their northern extremities all the 

 parallel divides curve round towards the west. Some other 

 cause must therefore be sought to account for the supposed 

 movement of the eastern flora southwards. It seems 

 probable that the real cause in this case was the advance 

 of the ice during the glacial epoch, driving the flora south- 

 wards and westwards, by which means not only were the 

 two separated Sino-Himalayan floras brought once more 

 into contact under new conditions, but apparently yet 

 another disturbing element added to further enrich the 

 growing flora of the parallel ridges. 



Baber, Johnstone, Wilson, and others have pointed out 

 the widespread glacial phenomena in Western China, and 

 Wilson ^ shows that the Chinese flora, the richest temperate 

 flora in the world, is more closely related to that of the 

 east coast of the United States than to that of the Eurasian 

 Continent. Thus it is evident that in China there has 

 actually been a movement of the flora westwards, and I 

 think it extremely probable that some portion of this 

 extra-continental flora reached the parallel divides, and, 

 mingling with the two halves of the old Sino-Himalayan 

 flora, travelled southwards, giving us the richest alpine and 

 mountain flora within the richest temperate flora in the 

 world, along the Burma-Yunnan frontier. For example, 

 Juglans and Magnolia, two typical genera of the Eastern 

 United States, are also common on the parallel divides. 



Let us now examine a single genus of plants and see 

 how far its distribution is accounted for on our theory 

 — namely, an original Sino-Himalayan range stretching 

 across uninterrupted to China, its continuity subsequently 

 broken by the pushing up of the parallel divides, thus 

 dividing the region into three great plant areas show- 



1 A Naturalist in Western China, by Ernest Wilson (London, 

 Methuen). 



