The Dinka is the largest, Nuer the second largest, the Zande the third-largest, and the Bari the fourth-largest of South Sudan's ethnic groups. They are found in the Maridi, Yambio, and Tombura districts in the tropical rainforest belt of Western Equatoria, the Adio of Azande client in Yei, Central Equatoria, and Western Bahr el Ghazal. In the 18th century the Avungara sib rose to power over the rest of Azande society, a domination which continued into the 20th century. British policies favoring Christian missionaries, such as the Closed District Ordinance of 1922 (see History of Anglo-Egyptian Sudan), and geographical barriers such as the swamplands along the White Nile curtailed the spread of Islam to the south, thus allowing the southern tribes to retain much of their social and cultural heritage, as well as their political and religious institutions.