include the pines, firs, yews, cypresses, araucarias, etc. Among them are to be found some of the largest forest trees, for instance the Mammoth Tree of California (Wellingionia gigantea), and the handsome Japanese Cedar (Cryptomeria japonica). On the other hand, the horsetails have dwindled down to a few insignificant species, although their forebears were great forest trees with wide-spreading branches. Also, their supposed precursors, the calamites, which grew in large groups on the margins of lagoons in the Carboniferous Age, and possessed huge jointed-stems, supported by massive rhizomes and far-spreading roots, appear to have died out altogether in the Permian period. Our modern ferns are also diminutive in comparison with those of former ages; while all the modern Lycopodiaceæ are puny representatives of the great tree-forms which flourished during the Carboniferous period, and which are now so largely met with as fossils in our British coal-fields.
Ice Age.—Among the physical phenomena which materially helped to mould Britain into its present shape was the incoming of the great Ice Age. This singular episode in the world's history lasted during the whole of the Pleistocene period, but not as one continuous span of advance and retreat of its accompanying load of ice, but rather as a series of ice ages alternating with warm intervals of long duration. At the time of maximum glaciation the larger portion of Britain was
