uninterrupted intercourse between remote parts of the Mohammadan world, with the ceaseless passage of travellers and scholars from the western limits of Europe on the one hand to the eastern limits of Asia on the other. This intercourse was quickened, as the Prophet had intended that it should be, by the institution of the annual pilgrimage. The mosque of Ibn Ṭulûn is not an isolated example of a direct borrowing by one region from another. The gates of al-Mehdiyyeh in Tunis were copied from the gates of Raqqah.[371] It is impossible to explain the curious niching of the walls of the eleventh-century palace of the Menâr, to take another Tunisian example, except by a comparison with the wall-surface decoration of Babylonia and Assyria.[372] I am fully aware that a long period of time had elapsed between the fall of the Mesopotamian empires and the erection of the Menâr, and that it would be vain to attempt to establish a continuous sequence of buildings between them, but I would point out that the Parthians, when they reconstructed the Babylonian palace at Tellôh, reproduced the Babylonian wall decoration so closely that de Sarzec was persuaded that the ruins of their palace belonged to the Chaldaean age.[373] Turn again to the fortress of the Bani Hammâd and you will find the cusp motive of Syria and Mesopotamia repeated on its arches;[374] and at the palace of Medînat al-Zahrâ in Spain (end of the tenth century) we have the plaster decorations of the walls of Sâmarrâ carried out in a style which betrays their Coptic and classical parentage,[375] though they are not devoid of characteristic motives, such as the palmette tree and the continuous pattern, which are rooted in oriental tradition.[376] In the same ruins the workers in stone have borrowed alike from Byzantium and from Mesopotamia; some of the continuous geometrical patterns are closely allied to those of Sâmarrâ,[377] while the free use of the crenellated motive may be compared with its use at Ukhaiḍir (Plate 87). The earliest Mesopotamian examples of such patterns as these are Parthian (Plate 86, Fig. 1).