Some writers, however, have argued the existence of extensive trade-relations from the occurrence of a peculiar kind of turquoise called _callais_ in some of the megalithic monuments of France and Portugal. The rarity of this stone has inclined some archaeologists to attribute it to a single source, while some have gone so far as to consider it eastern in origin. For the last theory there is no evidence whatsoever. No natural deposit of _callais_ is known, but it is highly probable that the sources of the megalithic examples lay in France or Portugal. It would of course be foolish to suppose that the megalithic people received none of the products of other countries, especially at a time when the discovery of copper was giving a great impetus to trade. No doubt they enjoyed the benefits of that kind of slow filtering trade which a primitive tribe, even if it had wished, could hardly have avoided, but they were not a great trading nation as were the Cretans of the Middle and Late Minoan Periods, or the Egyptians of the XIIth and XVIIIth Dynasties. We know nothing of their political conditions, of the groups into which they were divided, or the centres from which they were governed. That there were strong centres of government is, however, clear from the very existence of such huge monuments, many of which must have required the combined and organized labour of large armies of workers, in the gathering of which the state was doubtless strongly backed by religion. We have seen that the megalithic peoples frequently dwelt in huts of great stones. Yet in the majority of cases their huts must have been, like those of most primitive races, of perishable material, such as wood, wattle, skins, turf, and clay. As for their form there was probably a continual conflict between the round and the rectangular plan, just as there was in the stone examples. Which form prevailed in any particular district was probably determined almost by accident. Thus in Sardinia the round type was mostly kept for the huts and _nuraghi_, while the rectangular was reserved for the dolmens and Giants' Graves. Even here the confusion between the two types is shown by the fact that near Birori there are two dolmens with a round plan. Again, in Pantelleria the huts of the Mursia are rectangular, while the _sesi_, which are tombs, are roughly circular. It is therefore probable that the round and rectangular types of building were both in use among the megalithic people before they spread over Europe.