24 THE JOURNAL OF INDIAN BOTANY.
In other words, the breach in the Sino-Himalayan range is not the passive work of rivers cutting their way back, but is a far more formidable gap resulting from an active uplift of ranges whose axes cut across a possibly older uplift.
But whether we hold that this Sino-Himalayan flora was once more widely spread over Tibet at a time when that country had a more genial climate, as suggested in the first alternative ; or whether we hold that it merely crept across from Kauru to the Himalaya along a continuous range, prior to the uplift of the N.E. Frontier ran- ges, subsequently surging southwards through the breach they made, as in the second alternative ; we perceive that many species, now quite isolated, have survived unchanged through a long period, since they are now found in places as far apart as the Himalaya, N.E. Frontier and western China, to say nothing of Japan and N. America.
I say nothing as to the direction taken by the stream of flora, beyond the manifest fact that the alpine flora of the Htawgaw Hills (Lat. 26° N., Long. 98° 30 E.) has come neither from the east, nor from the west, but straight down the ranges from the north.
No doubt the flora of the great Asiatic divide has swayed back- wards and forwards, but in the main it is generally believed to have originated within the Arctic circle, and to have moved south- westwards into Asia and south-eastwards down the Atlantic coast of N. America, with the glaciation of the North. So much for its intro- duction into Asia ; but its movements after that are another matter, altogether. That hypothesis sufficiently accounts, in a broad way, for the relationship existing between the floras of the Himalaya, Japan and North America ; it by no means accounts for the relationship existing between the floras of the N. E. Frontier, the Himalaya, and Western China.
The Chinese element in the flora is also conspicuous, but a dis- tinction must here be made between the two sources from which it has been derived. These are : —
(i) The valleys to the immediate south and east; (ii) the high ranges of far western China, which, thrusting southwards from the main body hold in their grasp much of Yunnan. These sources must be carefully distinguished.
As regards the first, this flora extends eastwards no further than the Salween valley, and northwards no further than about the 26th parallel ; that is, it embraces the basin of the Shiveli river. It may be called the Burma-Yunnan area, and is characterised by many identi- cal species of 'Rhododendron, by the Candelabra Primulas, many Gelastraceae, Gesneraceae, etc. This flora is highly endemic. The typical Chinese flora is confined to higher altitudes, and includes
