18 TRANSACTIONS OF THE [S 



ESS. LXXX 



same flora, and consequently that any diflerences between 

 thera must have arisen since. The differences, however, 

 are marked and will have to be accounted for somehow, so 

 that we are no nearer a solution of the other problems, and 

 the theory will not account for certain peculiarities in the 

 distribution of the genus Primida. 



If then we reject the theory of a simultaneous origin for 

 these two floras, we must assume that they have mingled, 

 or successively originated from a common source : and 

 having satisfied ourselves that, under present conditions, 

 the Himalayan and Chinese floras are separated by im- 

 passable barriers, viz. the north-and-south-trending ranges 

 with deep arid valleys in between — it beingawell-established 

 fact that similarity of flora and fauna indicates not only 

 land connection, but in the case of plants the absence of 

 any great physical barrier such as a desert or high mountain 

 chain — we are justifled in assuming the previous existence 

 of a continuous range stretching from the north-west frontier 

 to well within China. This hj^pothetical range, the real 

 previous existence of which I shall endeavour to prove, will 

 in this paper be referred to as the Sino-Himalayan range, 

 while the flora of the Himalayas and of Western China will 

 be referred to collectively as the Sino-Himalayan flora. It 

 will be necessary to inquire in the first instance how this 

 range came to be so completel}^ severed by the north-and- 

 south-trending ranges already described. 



Retreat of the Ice : Climatic Changes. 



Leaving out of account the question as to how mountain 

 ranges are formed in the first instance, we shall see presently 

 reason to believe that these north-and-south-trending ranges 

 were thrust up subsequent to the uplift of the Sino-Hima- 

 layan range, interrupting its continuity ; and an examina- 

 tion of the floras of these parallel ranges will give a clue to 

 their mode of formation as an irruption area severing the 

 direct continuity of the Sino-Himalayan range. 



Comparing the floras of the Mekong-Sal ween and Mekong- 

 Yangtze divides, though separated only by the deep and 

 narrow Mekong valley, we find striking diflerences, not so 

 much in the floras themselves — though that too, especially 



