40 TRANSACTIONS OF THE [Sess. lxxx 



The following consideration will show that the flora of 

 the eastern range will soon lose many of its Himalayan 

 characteristics. The first hint of an irruption area break- 

 ing the continuity of the Sino- Himalayan range and trend- 

 ing from north to south would modify the distribution of 

 climate along that range east of the break, especially as 

 regards the monsoon ; it might still receive copious rain, 

 but its seasonable distribution would be different, since the 

 new ranges would to a large extent deflect the south-west 

 winds. The result would be a di.sturbance of the adjust- 

 ment reached by the ea.stern flora, with consequent variation 

 and redistribution till a new adjustment was arrived at, 

 and the point to which I would draw particular attention 

 is that, w^ith the irruption area acting as a channel of com- 

 munication southwards, two dif event floras will eventually 

 travel down it from the severed ends of the Sino-Himalayan 

 range, and, at least in the early stages, before deep dividing 

 grooves have been cut between the dividing ridges, come 

 into contact. The result would be. not only a new flora, 

 richer than either of its component streams, but a new 

 impetus to variation, partly owing to this mixing of types 

 and partly owing to the greater range of climate encountered 

 during a journey southwards from a continental towards 

 a maritime region — a range still further increased by the 

 retreat of the ice from the easternmost divide, as explained 

 at the beginning of this paper. 



Finally, with regard to the parallel ridges themselves, the 

 flora of the most easternly (the Mekong- Yangtze divide ^) 

 would bear less resemblance to the Himalaj^an flora than 

 does that of the most westernly (the Sahveen-Irrawaddy ^ 

 divide), for the reasons stated above ; the increased pre- 

 cipitation falling on the western rain screen as a result of 

 the new uplift seems to have given rise to the Irrawaddy, 

 thus draining the lake region and leaving behind the 

 plain of Hkamti. 



I have already remarked that the Himalayan flora 

 probably travelled south owing to the fact that the last 

 formed of the parallel ridges was alwaj^s in more or less 



^ There are mimeroiis north-and-soutli-trending ranges east of tlie 

 Yangtze and west of the Irrawaddy, but we are not concerned with 

 these just now. 



