24 SOCIAL SCIENTIST such occasion which has been immortalized in Islamic literature. Al-Isfahani,6 who devotes the eighth book of his, annals, which was finished in AD 961, to Himyarite kings, puts this event four hundred years before Islam. But, it seems, Yaqut comes nearer the truth when he assigns it to the reign of the Abyssinians. The breach apparently was then restored. But the final catastrophe alluded TO in the Koran (34:15) may have taken place afcer AD 542 and before 570 according to Philips Hitti.6 This breach dealt a severe blow to the sedentary civilization of southern Arabia. R A Nicholson says Mention has frequently been made of the bursting of the Dyke of Ma'rib, which caused an extensive movement of Yemenite stocks to the north. The invaders halted in the Hijaz, and, having almost exterminated the Jurhumites, resumed their journey. One groupy however, the Banu Khuza'a, led by their chief Luhayy^ settled in the neighbourhood of Mecca.7 In his correspondence with Marx, Engels has also taken note of this fact. He says. This artificial fertilization of the land which immediately ceased when the irrigation system fell into decay, explains the otherwise curious fact that whole stretches which were once brilliantly cultivated are now waste and bare (Palmyra, Petta, the ruins in the Yemen, districts in Egypt, Persia and Hindustan), it explains the fact that one single devastating war could depopulate a country for centuries and strip it of its whole civilization. Here too, I think, comes in the destruction of the south Arabian trade before Mohammed, which you very rightly regard as one of the chief factors in the Mohammedan revolution.8 J^ew Town on Trade Route This massive movement of population towards Hijaz, central Arabia and the north had its own economic repercussions. There were other factors, too. Due to constant friction between the Byzantine and Persian empires the trade routes were in a state of flux. It was ultimately the evolution of the trade routes that brought about zigzags and unpredictable fluctuations in the history of the Arabs. In the latter half of the sixth century A D, for instance, the Euphrates-Persian trade route, which had hitherto benefited from the commerce between the Mediterranean lands and the east, was encumbered and made dangerous by the constant friction between the Byzantine and Persian empires with concomitant tariffs^ political rivalries and general chaos. Egypt too was in a state of disarray, and, consequently, it could not have provided an alternative route. Businessmen had to take another peaceful although more difficult route leading from Syria down through western Arabia to the Yemenite ports that served the Indian trade. Yemen itself fell under foreign rule and by this time Palmyra and Nabatea in the north had disappeared. This was the right time for Mecca to fill a socio-economic vacuum thus created. Mecca