Page:The Journal of Indian Botany.djvu/445

DISTRIBUTION OF FLORAS IN S. E. ASIA. 23 Eastwards of the Mekong this Indo-Malayan flora spreads over the peninsulas of S. E. Asia, south of the tropic, and creeps north- wards up the China coast again almost as far as the Yangtze in latitude 30,° but only as a narrow belt. However, there is a wide divergence between the Indo-Malayan flora of Fokien and that of the Irrawaddy basin in the same latitude, owing to the N. E. Frontier barrier and the intervening plateau of Yunnan, which has prevented any transference of species across from west to east.

The Himalayan element is very strongly represented at interme- diate altitudes, less so in the alpine region, though the connection is well seen there too.

Now this flora cannot have made its way straight across the Assam and Irrawaddy valleys from the broken end of the Himalaya. It must have come right round the arc from the north ; and we naturally ask : How did it cross the gorge of the Dihang, which even now separates the eastern Himalaya from the N. E. Frontier belt ?

How can the seeds of alpines and sub-alpines, which are rarely constructed for long distance travel, cross such a chasm ? I need only instance such plants as Mcco?iopsis Wallichii, Isoyyrum adianti- folium and Lilium Thompsonianitm. Such plants as actually have seeds adapted to long distance travel, e.g. Podophyllum Emodi, Lilium giganteum and Rosa sericea, are found much further east than the N. E. Frontier ranges, extending into Kauru, Sbeuri, and often right across China to Japan.

Two possible explanations suggest themselves to account for this strong Himalayan element on the N. E. Frontier.

(i) That the whole region of southern and eastern Tibet enjoyed originally a temperate climate, and was covered by a uniform flora. When desert conditions began to prevail in Tibet, owing to the uplift, in Eocene times, of the Himalaya, and the N. E. Frontier ranges, the flora withdrew to these mountains, and was subsequently separated into three great blocks — Himalaya, N. E. Frontier, and western China— by the formation of the river gorges. This theory requires the flora to be of Pre-eocene age, but that is not impossible.

(ii) That there was previously a continuous Sino-Himalayan range, as it may be called, stretching across the present headwaters of the Irrawady and binding the Himalaya to the great China divide which passes through Kauru and Sbeuri ; and that this range was breached by the uplift of the N. E. Frontier ranges since Eocene times. But there is no proof at present that the N. E. Frontier ranges are younger than the Himalaya uplift though they certainly are features of original structure, and not, as Kropatkin believed, pidges separating grooves cut in the Tibetan plateau by the rivers,