But did all these animals exist at the same time? It is difficult to say. The study of the different gravels is most intricate—almost a special science in itself—in which but two or three men are adepts. It is hard, at first sight, to believe that the hippopotamus could have been the neighbour of the Arctic reindeer and musk ox: but that the woolly mammoth not only may have been such, but was such, there can be no doubt. His remains, imbedded in ice at the mouth of the great Siberian rivers, with the wool, skin, and flesh (in some cases) still remaining on the bones, prove him to have been fitted for a cold climate, and to have browsed upon the scanty shrubs of Northern Asia. But, indeed, there is no reason, _à priori_, why these huge mammals, now confined to hotter countries, should not have once inhabited a colder region, or at least have wandered northwards in whole herds in summer, to escape insects, and find fresh food, and above all, water. The same is the case with the lion, and other huge beasts of prey. The tiger of Hindostan ranges, at least in summer, across the snows of the Himalaya, and throughout China. Even at the river Amoor, where the winters are as severe as at St. Petersburg, the tiger is an ordinary resident at all seasons. The lion was, undoubtedly, an inhabitant of Thrace as late as the expedition of Xerxes, whose camels they attacked; and the ‘Nemæan lion,’ and the other lions which stand out in Grecian myth, as having been killed by Hercules and the heroes, may have been the last remaining specimens of that _Felis spelæa_ (undistinguishable, according to some, from the African lion), whose bones are found in the gravels and the caverns of these isles. And how long ago were those days of mammoths and reindeer, lions and hyænas? We must talk not of days, but of ages; we know nothing of days or years. As the late lamented Professor Sedgwick has well said:— ‘We allow that the great European oscillation, which ended in the production of the drift (the boulder clay, or till), was effected during a time of vast, but unknown length. And if we limit our inquiries, and ask what was the interval of time between the newest bed of gravel near Cambridge, and the oldest bed of bogland or silt in Cambridgeshire and Norfolk, we are utterly at a loss for a definite answer. The interval of time may have been very great. But we have no scale on which to measure it.’ Let us suppose, then, the era of ‘gravels’ past; the valleys which open into the fen sawn out by rivers to about their present depth. What was the special cause of the fen itself? why did not the great lowland become a fertile ‘carse’ of firm alluvial soil, like that of Stirling?