vegetation. The pine, or Scotch fir, buried in the oldest peat, gave place at length to the oak, and the oak, after flourishing for ages, yielded, in its turn, to the beech, the periods when these three forest trees predominated in succession tallying pretty nearly with the ages of stone, bronze, and iron in Denmark (p. 16). In the same country, also, during the stone period, various fluctuations, as we have seen, occurred in physical geography. Thus, on the ocean side of certain islands, the old refuse-heaps, or 'kitchen-middens,' were destroyed by the waves, the cliffs having wasted away, while, on the side of the Baltic, where the sea was making no encroachment, or where the land was sometimes gaining on the sea, such mounds remained uninjured. It was also shown, that the oyster, which supplied food to the primitive people, attained its full size in parts of the Baltic where it cannot now exist, owing to a want of saltness in the water, and that certain marine univalves and bivalves, such as the common periwinkle, mussel, and cockle, of which the castaway shells are found in the mounds, attained in the olden time their full dimensions, like the oysters, whereas the same species, though they still live on the coast of the inland sea adjoining the mounds, are dwarfed, and never half their natural size, the water being rendered too fresh for them by the influx of so many rivers.
As for several calculations, in which certain archæologists and geologists of merit have indulged, in the hope of arriving at some positive dates, or exact estimates of the minimum of time required for the changes in physical geography, or in the range and numerical preponderance of certain species of animals, or the advance in human civilisation in the Recent Period, or during the ages of stone, bronze, and iron, whether the computation related to the growth of peat, or to the conversion of water into land, since some lake settlements were founded, or the various depths at which, in the
