Secondly, the same eccentric fanaticism, which the Central Asiatics displayed when professing the doctrines of Zoroaster, has been the cause why their conversion to Islam cost the Arabs so much time and trouble. It took more than 200 years, before the religion of Mohammed had completely supplanted the old faith. No sooner had the conquerors left a town than the newly-converted inhabitants returned to their old faith, and the town had to be re-conquered and re-converted. But when the iron perseverance of the Arabs had at last succeeded in making them Mohammedans, they attached themselves to the new religion with the same fervour they had manifested in the old. As early as the beginning of the rule of the Samanides, we find in Transoxania men of high reputation, throughout Islam, for their learning and their exemplary piety. Belkh had already then acquired the name of Kubbetül Islam, the dome of Islam. The city and neighbourhood of Bokhara were crowded with the tombs of saints and learned men, and we can easily understand how it happened that these Turkestani cities had in piety and learning become successful rivals of Bagdad, the then centre of the Mohammedan world, where devotional zeal was eclipsed by the splendour of worldly grandeur. After the extinction of the dynasty of the Samanides, but especially during the Mongol conquests, no doubt all religious life suffered a temporary check, but the edifice has never been shaken to its foundations as in Bagdad, where Helagu, in destroying the phantom caliphate of Motasimbillah, broke the chief strength of Islam and scattered it to the winds. In Transoxania, on the other hand, its energies were being silently strengthened and matured. Timur aimed at making his native home the chief seat of Mohammedan learning, and his work was continued, though in a different spirit, by the rulers of the Sheibani dynasty. It can therefore excite no wonder that Bokhara has been able to preserve to the present day, that precise standard of religious asceticism which characterized Islam in the middle ages.