But one fact leads us to hazard a question. Not very far in the Bronze Age the first great wave of Celtic migration seems to have broken into northern Europe, as the Achæans had already found their way toward or into Greece. The Celts seem to have had their Vale of Avalon and Islands of the Blessed, whither the spirits of the departed migrated. We remember that when Ulysses went in search of the spirit of Achilles, and of other comrades in the war before Troy, he sought him in no underground world, but sailed far across the seas into the west. Such beliefs, and customs like incineration, are a slow growth, probably far older in origin than the Indo-European or Aryan migrations, of which some have thought them characteristic. May not this old and wide-spread belief be merely a continuance of views and conceptions already held by our Neolithic folk? We have already noticed the wide distribution of these megalithic structures.[92] They stretch along the shore of the Baltic, North Sea, and Atlantic Ocean down to the Mediterranean. Here they form a band along the south shore. We find them also in Soudan. In Egypt and Greece a far more precocious culture made it possible to replace them by pyramids and "treasure-houses." We find them in Palestine and farther eastward, along the Black Sea, and in India. In Europe they follow the coast lines, and do not seem to have been erected by the dwellers in the valley of the Danube. Their distribution is very similar to that of the great Mediterranean race and its extensions, but they extend far beyond the boundaries of any one tribe or people. They are the expression of a certain thought or conception which spread widely. It might be more correct to say that the general underlying conception was practically universal, but found expression in this form in one area, while in other regions it could not find this expression because conditions were unfavorable. It is exceedingly difficult to say just where the first dolmens were built. Opinions differ widely. They could have been built only in an area which had a fairly large and settled population who could unite in a large and difficult work, and had the means of carrying it out. The people were agriculturists who possessed no low grade of natural material or mental culture. Many such general considerations lead us to look for their first appearance somewhere in the region east of the Mediterranean, which was evidently the home of many other very ancient forms of culture.[93] CHAPTER VII NEOLITHIC INDUSTRIES