It seems strange that the plants above-named and many others should spring only from the dry sand of the dunes, but the apparent riddle is solved when we know that it is only the sand thus piled up, that becomes sufficiently warmed in the months of uninterrupted sunshine for these plants to flourish. Nowhere else throughout the tundra is this the case. Moor and bog, morass and swamp, even the lakes with water several yards in depth only form a thin summer covering over the eternal winter which reigns in the tundra, with destructive as well as with preserving power. Wherever one tries to penetrate to any depth in the soil one comes--in most cases scarcely a yard from the surface--upon ice, or at least on frozen soil, and it is said that one must dig about a hundred yards before breaking through the ice-crust of the earth. It is this crust which prevents the higher plants from vigorous growth, and allows only such to live as are content with the dry layer of soil which thaws in summer. It is only by digging that one can know the tundra for what it is: an immeasurable and unchangeable ice-vault which has endured, and will continue to endure, for hundreds of thousands of years. That it has thus endured is proved indisputably by the remains of prehistoric animals embedded in it, and thus preserved for us. In 1807 Adams dug from the ice of the tundras the giant mammoth, with whose flesh the dogs of the Yakuts sated their hunger, although it must have died many thousands of years before, for the race became extinct in the incalculably distant past. The icy tundra had faithfully preserved the carcase of this primitive elephant all through these hundreds of thousands of years.[8] Many similar animals, and others of a more modern time, are embedded in the ice, though it is not to be supposed that the tundra was ever able to sustain a much richer fauna than it has now. Bison and musk-ox traversed it long after the time of the mammoth; giant-elk and moose belonged to it once. Now its animal life is as poor and monotonous as its vegetation--as itself. This holds true, however, only with regard to species, not to individuals, for the tundra is, at least in summer, the home of numerous animals.