For agricultural purposes a soil must be drained of superfluous moisture. Hence practically all land below the altitude of 1000 ft. has been subjected to repeated drainage, after tree-felling, followed by cultivation and the enclosure of land.

All the causes at work too, in diminishing rare plants, accentuate this effect of drainage. The Fens are an illustration of the extent to which drainage has gone on within the last two centuries. All the tract from Peterborough and parts of Lincoln to Bedford and Huntingdon, and most of East Anglia, has undergone the most radical alteration. Where the Bittern boomed and the Ruff once bred, will be found acres and acres of potato fields. The reed swamps are gone, and so too is the marsh and bog vegetation.