DISTRIBUTION OF FLORAS IN S. E. ASIA. 23 



Eastwards of the Mekong this Indo-Malayan flora spreads over 

 the peninsulas of S. E. Asia, south of the tropic, and creeps north- 

 wards up the China coast again almost as far as the Yangtze in 

 latitude 30,° but only as a narrow belt. However, there is a wide 

 divergence between the Indo-Malayan flora of Fokien and that of the 

 Irrawaddy basin in the same latitude, owing to the N. E. Frontier 

 barrier and the intervening plateau of Yunnan, which has prevented 

 any transference of species across from west to east. 



The Himalayan element is very strongly represented at interme- 

 diate altitudes, less so in the alpine region, though the connection is 

 well seen there too. 



Now this flora cannot have made its way straight across the 

 Assam and Irrawaddy valleys from the broken end of the Himalaya. 

 It must have come right round the arc from the north ; and we 

 naturally ask : How did it cross the gorge of the Dihang, which even 

 now separates the eastern Himalaya from the N. E. Frontier belt ? 



How can the seeds of alpines and sub-alpines, which are rarely 

 constructed for long distance travel, cross such a chasm ? I need 

 only instance such plants as Mcco?iopsis Wallichii, Isoyyrum adianti- 

 folium and Lilium Thompsonianitm. Such plants as actually have 

 seeds adapted to long distance travel, e.g. Podophyllum Emodi, 

 Lilium giganteum and Rosa sericea, are found much further east 

 than the N. E. Frontier ranges, extending into Kauru, Sbeuri, and 

 often right across China to Japan. 



Two possible explanations suggest themselves to account for this 

 strong Himalayan element on the N. E. Frontier. 



(i) That the whole region of southern and eastern Tibet enjoyed 

 originally a temperate climate, and was covered by a uniform flora. 

 When desert conditions began to prevail in Tibet, owing to the uplift, 

 in Eocene times, of the Himalaya, and the N. E. Frontier ranges, the 

 flora withdrew to these mountains, and was subsequently separated 

 into three great blocks — Himalaya, N. E. Frontier, and western 

 China— by the formation of the river gorges. This theory requires 

 the flora to be of Pre-eocene age, but that is not impossible. 



(ii) That there was previously a continuous Sino-Himalayan 

 range, as it may be called, stretching across the present headwaters 

 of the Irrawady and binding the Himalaya to the great China divide 

 which passes through Kauru and Sbeuri ; and that this range was 

 breached by the uplift of the N. E. Frontier ranges since Eocene 

 times. But there is no proof at present that the N. E. Frontier 

 ranges are younger than the Himalaya uplift though they certainly 

 are features of original structure, and not, as Kropatkin believed, 

 pidges separating grooves cut in the Tibetan plateau by the rivers, 



