It was long since observed that the first homes of a dense population and a highly developed civilisation lay in fertile river valleys, such as those of the Lower Nile, the Euphrates, the Tigris, the Ganges, the Yang-tse-kiang. All these are situate in the hotter parts of the temperate zone; all are regions of exceptional fertility. The soil, especially when tillage has become general, is the first source of wealth; and it is in the midst of a prosperous agricultural population that cities spring up where handicrafts and the arts arise and flourish. The basins of the Lower Nile and of the Lower Euphrates and Tigris are (as respects the West Asiatic and Mediterranean world) the fountain-heads of material, military, and artistic civilisation. From them it spreads over the adjacent countries and along the coasts of Europe and Africa. On the east, Egypt and Mesopotamia are cut off by the deserts of Arabia and Eastern Persia from the perhaps equally ancient civilisation of India, which again is cut off by lofty and savage mountains from the very ancient civilisation of China. Nature forbade intercourse between these far eastern regions and the West Asian peoples, while on the other hand Nature permitted Egypt, Phœnicia, and Babylon to influence and become teachers of the peoples of Asia Minor and of the Greeks on both sides of the Ægean Sea. The isolation and consequent independent development of India and of China is one of the most salient and significant facts of history. It was not till the end of the fifteenth century, when the Portuguese reached the Malabar coast, that the Indian peoples began to come into the general movement of the world; for the expedition of Alexander the Great left hardly any permanent result, except upon Buddhist art, and the conquests of Mahmud of Ghazni opened no road to the East from the Mediterranean West. Nor did China, though visited by Italian travellers in the thirteenth century, by Portuguese traders and Jesuit missionaries in the sixteenth and seventeenth, come into effective contact with Europe till near our own time.