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VIII] hill-fortification. If an aquatic plant is fairly common on the continent near by, it is almost certain to occur in some isolated pond or river in the part of Britain opposite.

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