We must bear in mind that the means of transportation were few and inadequate. The wheeled cart appeared during the Bronze period, but we have no proof of its use earlier. The horse was not yet domesticated in Europe, and did not come into use in the Orient much before 2000 B. C.[107] Cattle may have been used as beasts of burden at an early period, but of this we know nothing. Roads of a certain kind, often probably hardly more than mere trails, almost certainly existed, especially in the neighborhood of the great stone monuments and larger villages. The great bar to free communication was the forest. To avoid this almost impassable barrier the roads and trails seem usually to have kept to the uplands, especially those where the chalk prevented a heavy forest growth. Certain river valleys, like that of the Thames, were heavily forested almost or quite to the shore, and hardly inhabited at this time. But when the forest drew back somewhat from the water's edge there was a most attractive place for human settlement. The river bottoms were fertile and easy of cultivation. There was grass for herds, wood for buildings and fuel. The rivers swarmed with fish down to recent times, and there was a great variety and abundance of smaller animal life. Such valleys formed natural routes of trade and migration.[108] We are not surprised to find that the earliest settlers of Sweden made their way from shore to interior along the rivers and lakes, whose shores are dotted with settlements of this age.[109] Déchelette tells us that this was true of the grouping of the Neolithic stations of France in three great provinces in the basins of the Seine, the Garonne, the Rhone, the Saone and the Loire. We remember the lake-dwellers. The valley of the Danube has been the great thoroughfare since the arrival of man in Europe. The great ancient civilizations of Egypt and Chaldea arose in the valleys of the Nile and the Euphrates. We know that the people of the shell-heaps must have ventured some distance from shore, fishing for cod. The transition from Paleolithic to Neolithic might almost be characterized as a time of change from a hunting life to one very largely of fishing. Long before this emigrants, probably from Asia Minor, had sailed out into the Mediterranean and settled Crete. Here, before 3000 B. C., a veritable sea-power had arisen carrying on trade with Egypt and the shores of the Ægean. The voyage of the Argonauts, a "much-sung" story and saga in Homer's time, may well have had a historical foundation in expeditions for trade and plunder along the shores of the Black Sea, up its rivers, and extending as far as distant Colchis. Hence the importance of Troy in ancient times and of Constantinople to-day.