If the view is correct, that a continuous growth of our flora, and to some extent of our fauna, takes place through transportation to our coasts, from which such species as can fight their way tend more slowly to spread inland, it seems to account for the present curious distribution of species, and this in a way that no continuous land-connexion will do.
As we have pointed out in a former chapter, the land-connexion across the North Sea was a wide alluvial plain and swampy delta. What use could dry-soil plants make of such a bridge? It would be no easier for them to cross than so much sea; and migrating mammals could not greatly help in the dispersal, where so many rivers had to be crossed. The aquatic species would be helped by such a connexion, and it is curious to note that several of our most interesting aquatic plants are confined to the eastern counties, which in post-glacial times had direct connexion with the delta of the Rhine, and probably with the Elbe.
Aquatic species, however, are not dependent on continuous waterways for their dispersal; they have great facilities for overleaping barriers and reaching isolated river-basins and lakes. Every dew-pond on the downs after a few years' existence contains aquatic plants and mollusca, and a still larger number of species, including fish, will be found in ancient flooded quarries or prehistoric dykes surrounding some
