2 TRAXSACTIONS OF THE [Sess. lxxx 



In highly mountainous regions it is often very difficult 

 to decide what is the dominant formation, or, more accu- 

 rately, to what single climatic formation the bewildering 

 series of small plant associations may be assigned. It is 

 only in the foot-hills that a mountain flora betrays not its 

 origin, which may be and generally is another matter, but 

 the dominant climatic formation in the midst of which 

 it is, as it were, a vast incident, or accident On the other 

 hand, when we come to consider a mountainous country the 

 size of Tibet, we are no longer justified in speaking of 

 an incidental formation in the midst of a dominant — the 

 incidental has become the dominant. Here physiographical 

 conditions, altitude and the trend of the mountains them- 

 selves, prevail, isolating a specialised climatic area which 

 bears no resemblance to surrounding climates ; and here we 

 find a new dominant formation with its own series of in- 

 cidental formations and plant associations. When, however, 

 we are dealing, not with a great elevated plateau like Tibet, 

 but with a high mountain range or series of ranges travers- 

 ing two or more climatic zones, the question is more difficult, 

 owing to the dovetailing of one flora into another, with 

 perhaps the introduction of a third flora which has found 

 its way along the range in some former period. In the 

 circumstances it is best to consider the flora of the mountain 

 range by itself, apart from the region in which it is 

 situated ; decide which is tlie dominant formation by 

 reference to the climate ; and resolve the incidental 

 formations from it. 



The state of afTairs alluded to is well illustrated on the 

 Yunnan-Tibet frontier, to a consideration of which this 

 paper is specially devoted, and I shall confine my remarks 

 chiefly to the flora of the Mekong-Salween and Mekong- 

 Yangtze divides, two great parallel mountain chains 

 separated by the deep and narrow Mekong valley. The 

 interest of this region — apart from the jumble of climates : 

 arid, monsoon, temperate, arctic, which succeed one another 

 rapidly in a vertical direction — lies in the fact that it is 

 the meeting-ground for several streams of vegetation. 

 There is, for instance, the Himalayan flora, which has 

 certainly travelled far eastwards into China and south- 

 wards into Burma ; the Chinese flora, which has^j-lso flowed 



