There are, however, in several bone-caverns certain peculiarities which demand a special explanation. Caves often contain large heaps of bones, situated at heights which it would have been absolutely inaccessible to the animals which lived in these places. How, then, was it possible that these bones could have found their way to such an elevated position? It is also a very strange fact, that no cavern has ever produced an entire skeleton or even a whole limb of the skeleton of a man, and scarcely of any animal whatever. The bones, in fact, not only lie in confusion and utter disorder, but, up to the present time, it has been impossible to find all the bones which in times past formed an individual. It must, therefore, be admitted, that the accumulation of bones and human remains in most of the caves are owing to other causes than the residence of man and wild animals in these dark retreats. It is supposed, therefore, that the bones in question were deposited in these hollows by the rushing in of the currents of diluvial water, which had drifted them along in their course. A fact which renders this hypothesis likely is that drift-pebbles are constantly found in close proximity to these bones. Now these pebbles come from localities at considerable distances from the cavern; often, indeed, terrestrial and fluviatile shells accompany these bones. It may sometimes be remarked that the femurs and tibias of large mammals have their points rubbed off, and the smallest bones are reduced to rounded fragments. These are all evident indications that these bones had been carried along by rapid currents of water, which swept away everything in their course; or, in other words, by the current of the waters of the deluge which signalised the quaternary epoch. During this period of the existence of primitive man, all these caverns were not applied to the same purpose. Some were the dens of wild beasts, others formed the habitations of man, and others again were used as burial-places. There is no difficulty in the idea that dens of wild beasts might very readily be occupied by man, after he had killed or driven out the fierce inhabitants; no discovery, however, has as yet confirmed this supposition. It can hardly be doubted that primitive man seldom dared to take up his abode in dens which had been, for some time, the refuge of any of the formidable carnivora; if he did, it was only after having assured himself that these retreats had been altogether abandoned by their terrible inhabitants. We shall now proceed to consider these three classes of caverns.