In endeavouring to trace out the early history of the human race we naturally turn our attention to all the means of investigation which either study or chance have placed at our disposal. Grottos and caves, the rock-shelters, the ancient camps, the centres of flint-working, the Scandinavian kitchen-middens, the _dolmens_, and the _tumuli_--all have lent their aid in affording those elements for the representation of the earliest epoch of the history of primitive man which we have already considered. The data which we shall resort to for delineating the bronze epoch will be of a different kind. Among all the sources of authentic information as to the manners and customs of man in his earliest existence, none, certainly, are more curious than those ancient remains which have lately been brought to light and explored, and have received the name of _lacustrine dwellings_. The question may be asked, what are these _lacustrine dwellings_, and in what way do they serve to elucidate the history of the bronze epoch? These are just the points which we are about to explain. The most important discoveries have often depended on very slight causes. This assertion, although it has been made common by frequent repetition, is none the less perfectly correct. To what do we owe the knowledge of a multitude of curious details as to pre-historic peoples? To an accidental and unusual depression of the temperature in Switzerland. But we will explain. The winter of 1853-1854 was, in Switzerland, so dry and cold that the waters of the lakes fell far below their ordinary level. The inhabitants of Meilen, a place situated on the banks of the Lake of Zurich, took advantage of this circumstance, and gained from the lake a tract of ground, which they set to work to raise and surround with banks. In carrying out these works they found in the mud at the bottom of the lake a number of piles, some thrown down and others still upright, fragments of rough pottery, bone and stone instruments, and various other relics similar to those found in the Danish peat-bogs. This extraordinary accumulation of objects of all kinds on the dried bed of the lake appeared altogether inexplicable, and every one was at fault in their remarks; but Dr. Keller of Zurich, having examined the objects, at once came to a right understanding as to their signification. It was evident to him that they belonged to pre-historic times. By an association of ideas which no one had previously dreamt of, he perceived that a relation existed between the piles and the other relics discovered in the vicinity, and saw clearly that both dated back to the same epoch. He thus came to the conclusion, that the ancient inhabitants of the Lake of Zurich were in the habit of constructing dwellings over the water, and that the same custom must have existed as regards the other Swiss lakes. This idea was developed by Dr. Keller in five very remarkable memoirs, which were published in German.[31]