The decadence of the nation was probably due to various causes. It is certain that the Arab tradition which sees in it the effect of a single catastrophe--the bursting of the dam at Marib, which was indispensable for regular irrigation--is far from being an adequate explanation. The bursting of the dam must itself have been the consequence of neglect on the part of a degenerate race. But there may well be some truth in the tradition, which connects the decline of this remarkable people, indirectly, at least, with the great migration of Yemenite tribes to the north. At that time--about the second century A.D.--a kind of retrograde movement seems to have set in throughout the civilisation of a large part of Arabia. At certain periods large numbers of Arabs had been able to write, at least in rude characters, as is sufficiently proved by numerous brief inscriptions; about the year 600 the art of writing in Arabia was the secret of the few. Even in Yemen tolerably trustworthy traditions of its palmy days survived only amongst individuals. The conquest of the country by the hated Abyssinians (525 A.D.) probably shattered the last remnants of national vigour, and the Persian conquest (about 570 A.D.) failed to quicken it afresh. It is true that the civilisation of Yemen was still superior to that of the rest of Arabia; for example, it carried on a fairly important manufacture of weapons and materials for garments. A dim consciousness still survived of great things that the country had wrought. But, since there were no historic records of such, the later Yemenites endeavoured to vindicate the fame of their forefathers by extravagant inventions and to show that they had done far greater deeds than were done by the Koreishites at the head of the Moslems.