It must be confessed that the question admits of no very striking rejoinder. The Mohammadan invaders were essentially nomadic; their dwelling was the black tent, their grave the desert sands. The inhabitants of the rare oases of western and central Arabia were content, as they are to-day, with a rude architecture of sun-dried brick and palm-trunks, unadorned by any intricate device of the imagination, and unsuited to any but the simplest needs. Even the great national shrine at Mekkah, the sacred house of the Ka’bah, was innocent of subsidiary constructions. It is true that on the northern trade-route the rock-cut tombs of Madâin Ṣâliḥ and of Petra bear witness to a higher order of artistic impulse, but it was an impulse which borrowed its power from without, from Hellenized Egypt and from Hellenized Syria. If there were an indigenous Arabian architecture worthy of the name, it can only have existed in the southern limits of the peninsula, where as yet exploration has been too imperfect to afford data for argument, nor is there evidence to show that in the seventh century of our era it can have played a part in the development of the northern tribes. Upon the northern frontiers the influence of the Byzantine and of the Sasanian empires would seem to have been predominant, and when the invaders established themselves in provinces which had been ruled from Constantinople or from Ctesiphon, they employed Greek and Persian artificers to fulfil their newly developed requirements and to satisfy their newly developed taste for architectural magnificence. The palaces of the conquerors were planned, constructed, and adorned by those whom they had conquered; their learning and their civilization were borrowed from them; even the ritual of their faith was shaped by contact with older forms of worship. No more significant example of the debt which Islâm owes to alien races can be cited than that which is afforded by the history of the mosque. Out of the mud-built courtyard of the Arab house, the open space for domestic and tribal assembly, Greek and Persian builders created an architectural type which governed the whole Mohammadan world. And the only contribution of the masters for whom they worked was the demand for just such large and open spaces, easily accessible, oriented in a certain manner, and partially shaded from the rays of the sun.