Here is another and a still stronger proof of the antiquity of the shell mounds. If we examine the shells themselves, we find that they all belong to still living species, and they are all exactly similar to such as might be found in the ocean at the present day. But it happens that this is not now the case with the shells of the same fish belonging to the Baltic Sea. For the waters of this sea are now brackish, and not salt; and since they became so the shell-fish in it have gradually grown smaller, and do not now attain half their natural size. The oyster, moreover, will not now live at all in the Baltic, except near its entrance, where, whenever the wind blows from the north-west, a strong current of salt ocean water is poured in. Yet oyster shells are especially abundant in the kitchen-middens. From all this we gather that, at the time of the making of these mounds, there must have been free communication between the ocean and the Baltic Sea. In all probability, in fact, there were a number of such passages through the peninsula of Jutland, which was consequently at that time an archipelago. * * * * * [Sidenote: The tumuli or barrows.] As ages passed on the descendants of these isolated fishermen spread themselves over Europe, and, improving in their way of life and mastery over mechanical arts, found themselves no longer constrained to trust for their livelihood to the spoils of the sea-shallows. They made lances and axes (headed with stone), and perfected the use of the bow and arrow until they became masters of the game of the forest. And then, after a while, man grew out of this hunter stage and domesticated other animals besides the dog: oxen, pigs, and geese. No longer occupied solely by the search for his daily food, he raised mighty tombs--huge mounds of earth enclosing a narrow grave--to the departed great men of his race; and he reared up those enormous masses of stone called cromlechs or dolmens--such as we see at Stonehenge--as altars to his gods.[9]