The dog was their only domestic animal, and their only arts the fabrication of rude pottery and implements of stone and bone, unless it can be inferred from the occasional presence of bones of cod and other deep-sea fish, that they possessed some form of boat or canoe, and had hooks and lines or nets. These mounds must have taken an enormous time to accumulate, for they are very numerous, and often of great bulk, some of them being 1000 feet long, 200 feet wide, and ten feet thick. How long such masses must have taken to accumulate must be apparent when we consider that the state of civilization implies a very scanty population. It has been calculated that if the neolithic population of Denmark required as many square miles for its support as the similar existing populations of Greenland and Patagonia, their total number could not have exceeded 1000, and each mound must have been the accumulation of perhaps two or three families. Ancient, however, as these mounds must be, they are clearly neolithic. They are sharply distinguished from the far older remains of the palæolithic period by the knowledge, however rude, of pottery and polished stone, and still more by the fauna, which is entirely recent, and from which the extinct animals of the quaternary period have disappeared; while the position of the mounds shows that only slight geological changes, such as are now going on, have occurred since they were accumulated. Similar mounds, on even a larger scale, occur on the sea-coasts of various districts in Europe and America, but they afford no indication of their date beyond that of great antiquity.