So long as these products of local industry were distributed by land, as in the case of Le Grand-Pressigny and Graig Llwyd, the old method of barter from tribe to tribe was possible and doubtless still continued. But when an island, such as Santorin, was the scene of production, such methods were ineffectual, and a definite organisation for export became necessary. To carry goods from one island to a neighbouring isle or to the mainland requires a ship and a crew, besides some representatives of the makers to effect the sales. When the ship has been equipped it is economical to provide a full cargo, and this would be more than one small community would need or could afford to purchase. This leads to trading voyages of some days’ or weeks’ duration, when the ship can call at a number of ports to meet the needs of many communities. The inland inhabitants have also to be catered for, and the most serviceable ports became in their turn fresh centres of distribution, and need a depôt under the supervision of a representative of the makers. Thus, even before the close of the stone age, we see developing, especially in the Mediterranean region, the beginnings of an organised commerce, involving visits paid by ships and their crews to distant ports and foreign communities, and sometimes leading to the establishment of small foreign trading settlements. With the introduction of metal these features increased rapidly, and, as we shall see, before the bronze age had been in existence for many centuries, an extensive trade had grown up, mainly by sea, but sometimes by land as well, so that bronze became known and used over most parts of Europe which were not too remote from the sea to be affected by sea-borne commerce. Thus a considerable mingling of peoples and cultures took place, not by the sudden arrival of large numbers of invading hordes, but by the constant infiltration of small bodies of merchants and seamen. The origin of the discovery of metal is still unknown, though many ingenious suggestions have been made. All investigators are agreed that gold, being the most strikingly conspicuous metal, was the first to be noticed and used, though there are those who believe that copper was almost if not quite as early a discovery. Professor Elliot Smith has made interesting suggestions in both cases. He believes that somewhere on the African shore of the Red Sea a cult arose which involved the use of the cowry shell as an amulet for fertility; such cults are well-known and widely spread.[82] For some reason the shells did not ultimately satisfy the people, or the supply diminished, and they made models in gold, deposits of which were found in that locality. Thus the virtue of the amulet, residing originally in its form, became transferred to the material, and gold became and has since remained a lucky and fortunate possession.[83]