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112 this difference is due to the agency of man, intentional to a certain extent, but mainly accidental, through the introduction of weeds and the preparation of the soil for crops. It must not be forgotten that man not only introduced the weeds, he prepared the land on which they could establish themselves, and from thence spread to uncultivated ground where few botanists now suspect that they are anything but "native."

In days when the people of Britain were hunters, the only extensive open country in the south and east seems to have been the chalk-downs and the sandy heaths. These were not suitable for new additions to the plant population, for the good land was all oak forest, the barren heaths were unfavourable for any but heath plants, and the alluvial flats were largely covered with sallow and alder. The open downs were clothed with close turf, and until this was broken by cultivation there would be little chance for migrants. It seems, therefore, that to obtain a clear idea of the plant population of this country before man's influence could be felt, we must study the flora of the submerged forests and of the associated alluvial detritus washed from the uplands during the same period. Till this is done more thoroughly, it is not much use to discuss what species are "native" and what "introduced"; the submerged forest will yield the answer to this question.