1915-16.] BOTANICAL SOCIETY OF EDINBURGH 39 



between which lies a new line of possible migration south- 

 wards. Eventually the new ridges might become severed 

 from both ends of the original range ; but, to begin with, 

 this is hardly possible, and the irruption area will be in 

 communication with at least one, and possibly with both, 

 ends of the broken range. At the present time there seems 

 to be no connection between the irruption area {i.e. the 

 Mekong- Yangtze, Mekong-Sal ween, and Salween-Irrawaddy 

 divides)and the broken ends of the supposed Sino-Himalayan 

 range (represented by the Himalayas in the west and the 

 Sin-ling and Pe-ling ranges between the Yangtze and 

 Yellow rivers in the east) ; but (|uite apart from the accept- 

 ance or rejection of the Sino-Himalayan range, it is 

 evident that there was once some sort of connection 

 between the Himalayas and at least the westernmost of 

 the parallel divides. Consider the first ridge thrown up 

 at right angles to the axis of the Sino-Himalayan range ; 

 it would maintain connection with the western half of the 

 broken range, if formed in the manner I have indicated, 

 and perhaps with the eastern half also. A second ridge 

 thrust up to the west of the first would have a twofold 

 ett'ect. It would, in the first place, be the natural channel 

 of communication between the Himalayas and the south, 

 thus taking the place of the first range which in time 

 would become isolated, being cut ofi" from both ends of the 

 broken range, and in the second place it would alter the 

 climate on the latter, and still more the climate further 

 east. Subsequent ridges pushed up in the west would 

 tend to emphasise these functions, so that the most 

 westernly- ranges would gradual!}" become the richest in 

 flora, both on account of being in communication with the 

 source of supply (the irruption area not having been yet 

 dissected by rivers) and owing to more favourable climatic 

 conditions. Thus we see that the flora of the Burma- 

 Yunnan frontier (Mekong-Salween and Salween-Irrawaddy 

 divides) would resemble the Himalayan flora more closely 

 than does the North China flora. ^ 



1 By the North China flora I mean that of the eastern half of the 

 old Sino-Himalayan range, the Sin-ling and Pe-ling ranges between the 

 Yangtze and Yellow rivers. The Himalayan flora is that of the western 

 halfr 



