38 TRANSACTIONS OF THE [Sess. lxxx 



ice and snow too supply a good deal of the deticit. We 

 are therefore justified in concluding that if the Himalayas 

 reached out into China, we should find a closely related 

 flora occupying its entire length ; the difl'erences might 

 be even less conspicuous than those between the N.W. 

 Himalayan flora and that of Bhotan to-day, as the con- 

 tinuous uplift of that lofty range has brought about changes 

 which in the esurly days of uplift would not yet have been 

 effected. There is no reason to suppose that any great 

 fluctuating movement of plants backwards and forwards 

 ever takes place ; on the contrary, all the north temperate 

 alpine floras at least seem to have invaded their present 

 homes from certain starting-points and then swept forward 

 the length of the range as though impelled from behind, 

 as indeed they often were by the advancing ice cap during 

 the glacial epoch. Thus it appears that a mountain range 

 is not occupied by plants in any haphazard fashion from 

 the surrounding country, but does actually fulfil its apparent 

 function as a transmitter of plants in one direction. 



Now the Himalayas trend about W.N.W. to E.S.E., and 

 it is probable that they received their present flora from 

 the N.W. at the time when the northern flora of Europe 

 was being driven southwards by the ice, for the Himalayan 

 flora is essentially European and Mediterranean ; and that, 

 owing to the prolongation of the Himalayas eastwards, 

 this flora, once establislied, would reach China. By the 

 time the vanguard liad travelled as far east as it could go, 

 so much time would have elapsed that many changes would 

 have taken place along the length of the range — the dis- 

 appearance of some species, the domination of others, and 

 so on ; in the meantime perhaps uplift has been going on, 

 and the rise of snow-clad portions of tlie range has cut it 

 up into watertight compartments, so to speak, separated 

 from each other by icy bulkheads between which tlie floras 

 must lienceforth develop independently. 



Now suppose the Sino-Hinialayan range cut clean across 

 in the manner alreadj^ described by an uplift at right 

 angles to its axis, as a result of which deep grooves are sub- 

 sc'juently trenched between the parallel divides by rivers 

 flowing between. At once the old Sino-Himalayan flora 

 is divided into two camps, an eastern and a western. 



