It has been suggested by some inquirers that the fashion of erecting such megalithic monuments of orthostatic blocks arose at one time and in one place, and was carried by degrees from centre to centre until it reached many widely scattered regions between Ireland and Polynesia.[131] It is not suggested that this culture, with which has been associated many others, such as terrace cultivation, irrigation, the use of conch shells and a number of others, was carried to all these places simultaneously or even within the same millennium, nor is it asserted that the people who introduced it to these widely scattered regions were of necessity the same. The idea may, I think, be better expressed by saying that a cult or religion became widely disseminated at an early date, that it developed many varieties in the regions in which it took root, and that these regions often became in time fresh centres for dissemination. Thus it might happen that a daughter cult might ultimately become spread through part of the region in which the parent cult had arisen. A parallel may be drawn from the spread of Christianity, especially in these islands. The new faith reached Britain during the period of the Roman occupation and thence spread to Ireland; later, when it had disappeared from the former, it passed from Ireland to Iona and thence back to England. We need not discuss the whole of this hypothesis, which is concerned with a much wider area than the lands we are considering. One of the most essential features, however, of this interesting thesis is that the people, whoever they were, who spread the cult of megalithic monuments and allied practices, were travelling in search of gold, silver, copper, tin, amber and pearls; they were, in fact, merchants in search of precious and easily portable commodities. Now Perry,[132] who has specially worked at this part of the hypothesis, maintains that megalithic monuments are invariably found in association with metalliferous deposits, amber coasts and pearl fisheries, and he has produced maps which appear at first sight very convincing. A careful examination of his megalith map shows that he has copied that of Fergusson, published in 1872,[133] and which represents far less accurately the distribution of these monuments than does that published by Colonel A. Lane-Fox in 1869.[134] Neither of these maps, however, gives us a really reliable summary of the facts. Much work has been done on this subject since these maps were produced, many fresh areas have been added, and two at least have been deducted; but no one has recently attempted to make a map of the European megaliths, or those of any country except Holland.[135] The French anthropologists have made a list of the dolmens in France, and published a summary giving the number noted in each department,[136] a catalogue of the British megaliths is in process of formation.