The neolithic period which preceded that of metals is of longer duration, but still comparatively recent. Attempts have been made to measure it by a sort of natural chronometer in the case of the lake-villages, by comparing the amount of silting up since the villages were built with the known rate of silting up since Roman times. The calculations vary very much, and can only be taken as approximative; but the oldest dates assigned do not exceed 5000 B.C., and most of them are not more than 2000 or 3000 B.C. It must be remembered however that the foundation of a lake-village on piles implies a long antecedent neolithic period, to have arrived at a stage of civilization which made the construction of such villages possible. This civilization coincides wonderfully with that of the primitive Aryans as shown by linguistic palæontology. The discussion as to the origin of the Aryans has thrown a great deal of light on this question, and has gone far to dispel the old notion that they radiated from some centre in Asia, and overran Europe in successive waves. On the contrary, all the evidence and all the best authorities point to their having occupied, when we first get traces of them, pretty much the same districts of the great plain of Northern Europe and Southern Russia as we now find them in, and developed there their distinct dialects and nationalities; while the words common to all or nearly all the Aryan families point to their having been pastoral nomads, in a state of civilization very like that of the earlier lake-villagers, before this separation took place. The Scandinavian kitchen-middens, or shell-mounds, carry us further back into this early neolithic period. The shell-mounds which are found in great numbers along the Baltic shore of Denmark are often of great size. They are formed of an accumulation of shells of oysters, mussels, and other shell-fish, bones of wild animals, birds, and fish, all of existing species, with numerous implements of flint or bone, and occasional fragments of coarse pottery. They are decidedly more archaic than the lake-dwellings, showing a much ruder civilization of savages living like the Fuegians of the present day, in scanty tribes on the sea-shore, supported mainly by shell-fish, supplemented by the chase of wild animals.