Arab immigration into neighbouring territories to the north of their peninsula had started many centuries before Muhammad, and the conquest. The coming of Islam was accompanied by a development of urbanization. In the economic-social structure, the principal distinction to be noted is that between the town and the countryside. The medieval Muslim world was situated almost exclusively within the subtropical zone. The agriculture of the Muslim countries has given rise to a special literature, the forerunner of which appeared in Iraq the 'Nabataean agriculture' of Ibn Wahshiyya, a mixture of oral traditions and borrowings from ancient treatises. In the Abbasid period, the great centre for the whole of the East was Baghdad, to be replaced after the fifth/eleventh century by Cairo, while the distant countries of the Muslim West also had their own activities, though on a smaller scale. In the Umayyad period, the governmental and administrative institutions were relatively simple.