This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The origin of the present meadow plants must be sought in the far past. Apart from the derivation of the vast bulk of meadow and pasture plants in the way described, i.e. after tree felling, cultivation, drainage, enclosure, etc, there were originally certain areas where trees did not grow, usually at high altitudes or near the coast, where in the distant past the forerunners of the plants of the present open spaces were already evolved.
Certain of these northern plants are common associates of the wet meadows, such as the White Clover, Red Clover, Meadow Sweet, etc, and many have been found in beds of Preglacial and later ages. In each area the river valleys of a district should be carefully investigated to obtain traces of seeds to determine what was the type of vegetation in each district.