THE NUER OF THE SOUTHERN SUDAN      281
travellers are to be trusted, to have been much change of territory during the last fifty years. This eastwards migration is a fact that has to be taken into account, with those related earlier, if we wish to know why the eastern tribes are larger, territorially and numerically, than the western tribes, for it may be assumed that the struggle of conquest and settlement, and absorption of Dinka on an unprecedented scale, had some effect on the migrating hordes.
To the north, the Nuer are in varying degrees of contact with Arabs, the peoples of the Nuba Hills, the powerful Shilluk kingdom, and certain small communities in Darfung (Burun and Koma); while to the east and south-east they are bordered by the Galla of Ethiopia, the Anuak, and the Beir. Wherever the Nuer have direct relations with these peoples, they are hostile in character.
Arab slave-raiders from the Northern Sudan intruded here and there into the more accessible portions of Nuerland in the second half pf the nineteenth century, but nowhere did they gain the upper hand or, indeed, make a marked impression on the Nuer, who opposed them as strongly as they resisted later the Egyptian Government, which undertook no serious operations against them. The Nuer likewise treated British rule with open disrespect till, as a result of lengthy military operations between 1928 and 1930, their opposition was broken and they were brought under effective administration. With the exception of this last episode in their history, the Nuer may be said to have reached in their foreign relations a state of equilibrium and of mutual hostility which was expressed from time to time in fighting.
A tribe is divided into territorial segments which regard themselves as separate communities. We refer to the divisions of a tribe as primary, secondary, and tertiary tribal sections. Primary sections are segments of a tribe, secondary sections are segments of a primary section, and tertiary sections are segments of a secondary section. A tertiary section is divided into villages and villages into domestic groups. A member of Z2 tertiary division of tribe B sees himself as a member of Z2 community in relation to Z1, but he regards himself as a member of Y2 and not of Z2 in relation to Y1. Likewise, he regards himself as a member of Y, and not of Y2, in relation to X. He regards himself as a member of tribe B, and not of its primary section Y, in relation to tribe A. Thus, on a structural plane, there is always contradiction in the definition of a political group, for a man is a member of it in virtue