Let us now take a general survey of the results of our observations. First of all it is apparent that the doctrine of a primal convention with regard to the use of any one particular article as a medium of exchange is just as false as the old belief in an original convention at the first beginning of Language or Law. Every medium of exchange either has an actual marketable value, or represents something which either has or formerly had such a value, just as a five-pound note represents five sovereigns, and the piece of stamped walrus skin formerly employed by Russians in Alaska in paying the native trappers represented roubles or blankets[79]. To employ once more the language of geology, we have found evidence pointing to certain general laws of stratification. In Further Asia we have found a section which presents us with an almost complete series of strata, whilst in other places where we have been only able to observe two or three layers, we have nevertheless found that certain strata are invariably found superimposed upon others, just as regularly as the coal seams are found lying over the carboniferous limestone. As soon as the primitive savage has conceived the idea of obtaining some article which he desires but does not possess by giving in exchange to its owner something which the latter desires, the principle of money has been conceived. Shells or necklaces of shells are found everywhere to be employed in the earliest stages. When some men began to make weapons of superior material, as for instance axes of jade instead of common stone, such weapons naturally soon became media of exchange; when the ox and the sheep, the swine and the goat are tamed, large additions are made to the circulating media of the more advanced communities; then come the metals; the older ornaments of shells and implements of stone are replaced by those of gold (and much later by silver) and by weapons of bronze as in Asia and Europe, and by those of iron in Africa. Copper and iron circulate either in the form of implements and weapons, such as the axes of West Africa, the hoes of the early Chinese and modern Bahnars, and the ancient Chinese knives, all of which remind us of the axes and half-axes in Homer; or in the form of rings and bracelets, like the manillas of West Africa and the ancient Irish fibulae; or else in the form of plates or bars of metal, ready to be employed for the manufacture of such articles, as we saw in the case of the iron bars of Laos, the iron discs of the Madis, and the brass rods of the Congo. Again we are reminded of the mass of pig-iron, which Achilles offered as a prize[80].