As it ebbed, the new cities of Genoa, Pisa, and Venice began to capture the trade and hold the control of the sea that once had been Saracen, until the Christian control was so well established as to make possible the Crusades. Later, as we shall see, a second invasion of Mohammedans, the Turks, ably assisted by the descendants of the Arabs who conquered Spain, once more threatened to control the Mediterranean for the cause of Islam. But the Persian Gulf and the Indian Ocean, which fell into the hands of the Arabs as soon as they took to the water, remained in Arab hands down to the times of the Portuguese. In those waters, because they were cut off from the Mediterranean, the Saracen had no competitor. As early as the eighth century Ceylon was an Arab trading base, and when the Portuguese explorers arrived at the end of the 15th century they found the Arabs still dominating the water routes of India and Asia, holding as they had held for seven centuries a monopoly of the commerce of the east. Of the Mediterranean during the struggle between Christian and Saracen a recent English writer makes the following suggestive comment: "The function of the Mediterranean has thus undergone a change. In early times it had been a barrier; later, under the Phoenicians, it became a highway, and to the Greeks a defense. We find that the Romans made it a basis for sea power and subdued all the lands on its margin. With the weakening of Rome came a weakening of sea power. The Barbary states and Spain became Saracen only because the naval power of the eastern empire was not strong enough to hold the whole sea, but neither was the Saracen able to gain supreme control. Thus the conditions were the same as in the earlier days of the conflict between Rome and Carthage: the Mediterranean became a moat separating the rivals, though first one and then the other had somewhat more control. The islands became alternately Saracen and Christian. Crete and Sicily were held for centuries before they were regained by a Christian power."[1] [Footnote 1: GEOGRAPHY AND WORLD POWER, Fairgrieve, p. 125.]