The Land of Deep Corrosions 271 
In this connection it must be remembered that our high 
mountain chains are in direct communication with the 
Tibetan plateau and form ideal lines of migration. 
If now we compare the flora of the Mekong-Salween 
divide with that of the Mekong-Yang-tze divide, we arrive 
at an apparently anomalous conclusion, for a_ traveller 
crossing these two watersheds in the summer would almost 
unhesitatingly pronounce the alpine flora of the Mekong- 
Salween divide to be the richer of the two. Yet I do not 
believe this first impression would be correct, for closer 
examination seems to me to reveal the fact that, while the 
Mekong-Salween divide is richer in genera, the Mekong- 
Yang-tze divide is richer in species, but that there is little 
difference in the sum total of species on the two ridges. 
Even this, however, might seem to argue against the 
Mekong-Salween ridge having come into prominence 
subsequent to the Mekong-Yang-tze ridge, since it requires 
a longer period of time for a varying group of plants to 
acquire generic rank than it does for them to be classed as 
distinct species ; but it is quite a gratuitous assumption to 
suppose that they have attained generic rank zm” sztu, 
whereas there is every reason from the nature of the case 
to think that the high alpine flora of the Mekong-Yang-tze 
divide has greatly enriched itself 2 sz¢z. 
Again, if the Mekong-Yang-tze watershed did actually, 
ages ago, receive a greater rainfall which has since been 
reduced, it is natural to suppose that many plants adapted 
to flourish in an extremely wet climate would by now have 
disappeared or become greatly reduced in numbers; and 
the few plants to which I paid attention, common to the 
two mountain chains, rather bear out this supposition by 
their distribution. 
The sulphur-yellow JZeconopsts integrifolia for example, 
a plant which flourishes in a wet climate, occurs very 
sparsely on the Mekong-Yang-tze ridge, being confined to a 
few favourable localities, whereas whole meadows of it are 
to be seen on the Mekong-Salween ridge. The plant is 
very common to the north and east, on the Ssii-chuan 
mountains, whence it is reasonable to infer that it has come 
down these ridges, having since almost disappeared from 
the Mekong-Yang-tze divide, though it is of course possible 
