272 The Land of Deep Corrosionus 
that we find there, not a remnant of a former extensively 
distributed species, but a plant which is trying to establish 
itself. Such cases suggest lines of investigation, but require 
a much wider acquaintance with facts to justify conclusions 
of any importance 
It is significant that while those plants common to both 
divides are unusually rare on the Yang-tze side and extremely 
abundant on the Salween side of the Mekong, this difference 
tends to disappear the nearer we approach the limit of 
plants (which, by the way, is very high on the Mekong- 
Yang-tze watershed) for the reason that during the rainy 
season there are continuous mists hanging over the peaks, 
sufficient to keep the atmosphere saturated even when no 
rain is actually falling, while the heavy dews experienced at 
great elevations may also have something to do with it. 
Thus several species of Primula (e.g. P. bella) occur 
almost as commonly at 15,000—16,000 feet on the Mekong- 
Yangtze divide as they do at 13,000—14,000 feet on the 
Mekong-Salween divide, which again suggests not only 
that the flora was originally the same on both sides (and 
certainly it would be a most remarkable circumstance if it 
were not!), but also that it is the rainfall which has largely 
determined existing differences in the flora of the two ridges ; 
such differences being far more noticeable in the forest 
belt, the lower limit of which is defined entirely by the 
rainfall, than in the alpine belt. 
Similarly the very conspicuous fact that the flora of the 
Mekong-Salween divide is essentially a summer flora, 
attaining its maximum development during the months of 
June and July, while that of the Mekong-Yangtze divide is 
equally an autumn flora, attaining its maximum development 
between August and October, may be attributed to the 
seasonal distribution of rain in the two cases, as well as to 
the fact that snow is beginning to fall on the Mekong- 
Salween divide early in October. 
The comparative preponderance of species over genera 
on the Mekong-Yang-tze divide, especially in the two genera 
Saxifraga and Gentiana, which occur in enormous numbers, 
may be adduced as evidence for a changing climate. A 
very large extent of territory is available to plants on the 
one watershed which is denied them on the other, and one 
