The inhabitants of the caves and rock-shelters did not clean house every spring and fall, or, indeed, at any other time; the refuse and debris of the household were allowed to accumulate upon the floors of the caves and rock-shelters, and to this we owe nearly all the knowledge possessed by civilized man of the domestic arts, weapons, food, etc., and the general conditions under which the cave-men lived, as well as of the animals which were their contemporaries. The floors of these ancient dwellings, when excavated by scientists, show several feet of debris or accumulations, which are called "brecchia," being a conglomeration of dirt, bones of animals, bones of human beings, weapons, implements and other artifacts, which are frequently cemented with limestone formations caused by the drippings of the rocks and caves, in the nature of stalactites and stalagmites. Not only have we learned from this "brecchia" what progress the cave-men had made in domestic art, but our knowledge of the animals which lived in the locality and were their contemporaries is almost wholly derived from rude pictures made by these cave people, who seem to have suddenly developed an artistic sense and made such pictures by etching or scratching them with sharp flints upon pieces of bone, ivory and slate. These drawings are by no means so crude and wanting in artistic skill as we would be inclined to expect. The animals depicted are readily recognizable; such drawings show groups of reindeer, now found only in the arctic regions; the wild horse; the single horned rhinoceros; the giant elk; and on a smooth piece of his own tusk, we see the curved-tusked, hairy elephant of gigantic size--the mammoth, or _elephas primigenius_, whose bones have been found in many parts of Europe and Asia, and of which at least one specimen was found whole with the flesh intact, in the frozen tundra of Siberia. But for these drawings--the natural history of his time--left by cave-men, we would not know that immense animals, now long extinct in Europe, had contested with men of the cave period, the ownership of the forests, swamps, plains and mountains. In the "brecchia" of these caves, are often found long bones of animals which have been split longitudinally to obtain the marrow, which was regarded as a great delicacy by primitive man; and as some long bones of the human body have been found split in the same manner, some scientists have concluded that cave men were cannibals, or at least occasionally made a feast upon the bodies of prisoners captured in war, or upon such sacrifices when offered to the gods.