It is a mistake to suppose that all the bones of all the men and animals which have lived on the earth’s surface are naturally and as a matter of course permanent enduring things. On the contrary, when they are buried in soil or sand permeated by water, they slowly soften and decay, dissolve and disappear. When washed into streams and rivers or into the sea, they break up and dissolve. No bones were dredged up from the floor of the ocean by the explorers of the Challenger expedition. A bone sunk in the sea gradually dissolves. Only those bones (and the same is true of shells) are permanently preserved which happen to get into certain favourable positions, embedded in clay or hard deposit, which is not disturbed, and becomes slowly raised up and free from soaking water before the bone is dissolved; or, again, those which have been protected in the accumulated deposits of the floor of a cavern covered in by layers of hard calcareous slab or stalagmite, which usually is formed by the water dripping from the limestone roof and walls. The limestone is dissolved like sugar, and is deposited when the water evaporates—“petrifying” the floor of the cave. It is owing to this rarity of the natural preservation of bones that we never find more than a few of those of extinct animals of various degrees of antiquity, and never more than a very few of those of the ancient men who lived in caverns and made “flint implements.” [Illustration: FIG. 66.—An unpolished but beautifully chipped flint knife, of the Neolithic Age, from Denmark. (This figure and Fig. 67 are from the guide to the antiquities of the Stone Age in the British Museum).]