Many of our peculiar mollusca and plants are limestone species, which must have crossed over at a single leap, for no elevation or depression will connect the various isolated limestone masses of Britain. A post-glacial elevation would connect the North Downs with the corresponding chalk-hills of France; but these Downs are isolated by wide tracts of non-calcareous strata from the areas of Oolite or Carboniferous limestone to which many of our limestone animals and plants are now confined. There is also nothing in the present distribution of our limestone species to suggest that any great stream of migrants used this bridge of chalk-downs. It may be asked, Why discuss these questions here, if all these peculiar species are unknown in the submerged forests? In certain cases negative evidence is of great value, and the deficient flora of the submerged forests is a case in point. We find a striking contrast between this ancient flora and the flora which flourished when cultivation of the land had begun. The Roman deposits in Britain yield many species which have not yet been found in the submerged forests, and even the earlier Celtic deposits have already yielded a few of them. To a large extent 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.