26 TRANSACTIONS OF THE [Sess. lxxx 



Salween glaciers have not retreated far, and the floras are 

 practically the same to this day. At Hpiniaw on the 

 Salween-Irrawaddy divide (latitute 2(')^) not only was the 

 o-eneral fades of the alpine flora the same as that met with 

 at Doker-la (Mekong-Salween divide, latitude 28"), but 

 many of the species were identical: ('jj. Polygonum ker- 

 mesiiium, Ward mss., R/uxIodendron sp. with " black " 

 (port- wine) flowers; Orclris Chusna, Don, var. : Androsace 

 gf rant I folio, Watt ; besides species of Primula (§ Omj)halo- 

 gramnia, § Bdlo )/f/ialictrnm , Creman thodiuni, Mecono})sit<, 

 Saxifrugo. Thus it woidd seem certain (i) that these three 

 parallel ranges once had the same flora which, derived from 

 a single source, travelled down the ranges from the north 

 and west (whither all three ranges turn later) and became 

 diflerentiated at a later date owing partl}^ to (ii) the west- 

 ward retreat of the monsoon which was cut ofl' from tlie 

 east by the gradual elevation of the western ranges, and 

 interpolation of more and more mountains, (iii) that in this 

 wa}' two climates, a monsoon climate west of the Salween 

 and a warm temperate climate east of the Yangtze, became 

 shar])ly dcflned and separated from each other. 



Ijtsjx'fsod of Seeds. 



Except under accidental circumstances, the several valleys 

 separating the parallel ranges constitute physical barriers 

 to the spread of plants east or west from one range to 

 another, for the Salween valley north of the Ka'-gur-pw 

 uplift and the Mekong valley tin-oughout its length are 

 extremeh' arid, and the furthci' oiu; traces them towards 

 their respective sources tlie more arid do they become. 

 However, we have just seen that l:»eyond Ka'-gur-pw the 

 flora on the Mekong-Sal ween and Mekong- Yangtze divides 

 is identical, both in the forest and alpine belts, so that we 

 cannot doul>t the commoii source to which both ultimately 

 luid access. The only means by which seeds could be trans- 

 ferred directl}^ from one range to another would be (i) wind, 

 and (ii) birds. As regards wind, seeds capable of being 

 carried long distances by wind, e.g. those of Compositae, 

 Clematis, etc., might be so transferred from range to range, 

 and there are species of ( 'Icnoil is (e.g. (J. 'moidoiia, Ham. ) and 



