3?6	AFRICAN POLITICAL SYSTEMS
pasturage, and it is possible, therefore, for village communities of the rains to maintain a relative isolation in the drought. But where, as in the Lou tribe, for example, scarcity of water and pasturage compels more extensive movement and greater concentration, people who are very widely distributed may have more social contact with one another than is the case in western Nuer-land. The isolation and autonomy of local communities are broken up by economic necessity and the size of the political group is thereby enlarged. This fact has to be considered in relation to the further fact that to the east of the Nile wider stretches of elevated ground allow larger local concentrations in the rains than is usual to the west of that river. Moreover, seasonal concentration offers an explanation, though by no means a full one, of the location of tribal boundaries, since they are determined not only by the distribution of villages, but also by the direction in which the people turn in their move to dry season pastures. Thus the tribes of the Zeraf Valley fall back on the Zeraf River and therefore do not share camps with the Lou tribe, and that part of the Lou tribe which moves east and north-east make their camps on the Nyand-ing River and on the upper reaches of the Pibor and do not share their waters and pasture with the Jikany tribes, who move to the upper reaches of the Sobat and the lower reaches of the Pibor, Furthermore, that some of the larger Nuer tribes are able to preserve a degree of tribal unity without governmental organs may in part be attributed to seasonal migration, since, as explained above, the different local sections are forced by the severity of the latitude into mutual contact and develop some measure of forbearance and recognition of common interests.
Likewise, a tribal section is a distinct segment, not omy because its villages occupy a well-demarcated portion of its territory, but also in that it has its unique dry-season pastures. The people of one section move off in one direction and the people from an adjoining section move off in a different direction. Dry-season concentrations are never tribal, but always sectional, and at no time and in no area is the population dense.
The total Nuer population is round about 300,000. I do not know the total square mileage of the country, but to the east of the Nile, where there are, on a rough estimate, some 180,000 Nuer, they are said to occupy 26,000 square miles, with the low density of about seven to the square mile. The density is probably no