A glance at the archaeological literature of the last dozen years demonstrates all too clearly that the popularity of the 'invasion hypothesis' in Irish archaeology is quite undiminished. An almost incessant stream of immigrants appears to have tramped ashore from the Mesolithic period to the Iron Age. Even reckoning those pre-eminent invaders, the Beaker Folk, as merely a single influx, over a dozen significant prehistoric population movements are claimed by a variety of writers. The general picture presented suggests that Ireland throughout much of her prehistory was, if not an archaeological Ellis Island, at least a desirable landfall for the land-hungry, the dispossessed and the adventurous of most of the rest of Western Europe. Major changes and innovations in the archaeological record-in monument or artifact typemay conceivably be the result of either independent invention, or diffusion or of a combination of the two. The occurrence of megalithic tombs in very different cultural and chronological contexts in Western Europe, India and Japan, for example, is an instance of independent invention.

Our knowledge of the obsidian trade makes it an unique phenomenon in the prehistory of the Middle East. We know more about it than about any comparable exchange network. A great deal of work has now been done on the chemical composition of obsidian and on the location of its sources, on quantitative analyses, and on the spatial distribution of the material. The two major areas of supply, Central Anatolia and the area west of Lake Van have been extensively explored and individual sources pinpointed. It has been possible to reconstruct the movement of obsidian to sites as far away as Beidha in South Palestine and to Ali Kosh in South-West Iran. The distances involved are well over 1,600 km as the crow flies. Comparatively little work has been done on the mechanism of the trade, on the processes by which the obsidian changed hands, probably because such processes are difficult and sometimes impossible to determine from the archaeological record. In order to attempt such a task it is necessary to broaden our conceptual basis for studying data bearing on trade. To quote Adams (1974, 241) 'What is needed for this broadening to occur is a much more substantial awareness of ethnohistoric, historic and ethnographic studies of trade.