In the Ultra-Indian peninsula different layers of nationalities may be distinguished, being successive arrivalsfrom different quarters. In most cases it is difficult to determine with exactitude the time of their respective settlements; but, as a general rule, the later conquerors have driven the former possessors of the soil into the hills, appropriating to themselves the more fertile lands on the banks of the rivers; and it may be readily assumed that the inhabitants we now find in the valleys of the Menam and Irawaddi are the most recent immigrants. History shows this to be the case. Whenever, in the course of events, the empires stretching along the great rivers became weakened by luxury or broken up by internal dissensions, the rude and warlike tribes of the surrounding mountains, watchful of the growing weakness of their former masters, and the decay of their defences, have burst from their forests and made slaves of the populations of the towns. This phenomenon, not unknown in other parts of the world, has been frequently repeated in the histories of Assam and Burmah, of Siam and of Kambodia; and it is by keeping before our eyes these continual revolutions that we may collect the disjecta membra of arts and sciences from the different quarters to which they have been scattered, and arrange them under one comprehensive aspect, where each of them occupies its natural position.