As we have seen in some earlier chapters, distribution of certain types of bronze implements may be taken as evidence of trade, and we have to consider whether it was some form of commerce which carried the leaf-shaped swords to Ireland, Finland, Podolia and Egypt, or whether this wide distribution betokens some other form of movement. Before the days of fairly large ships and highly organised industry it is not uncommon to find implements, whether of flint or obsidian, copper or bronze, carried from country to country, without apparently any general movement of the population. On the other hand, when pottery and heavier or more easily damaged goods pass from one centre to another, it usually betokens migration. We have seen how this was so when the beakers were carried from Bohemia towards Jutland and Britain. Of course Roman pottery was shipped extensively for trade purposes, as were red figure vases and other types of Greek ceramic wares. The same is true, though to a less extent, of Mycenean and some Minoan wares, for the Ægean traders exported oil and wine. But such export of pottery betokens a relatively high civilisation and a well-organised commerce. Under more primitive conditions we may, I think, postulate that where metal implements or small cult objects alone were carried, these are evidence only of trade, while when pottery is found, as it were, on the move, this indicates a movement of the potters, hence a migration of people. When pottery and weapons are both found moving together, especially if the weapons are of a more advanced type than those hitherto found in the land into which they are being introduced, we may suspect, if indeed we cannot be sure, that we are dealing with a hostile invasion and the arrival of conquerors.