284 AFRICAN POLITICAL SYSTEMS
segments or with two tribes. The tribal system as defined by sociological analysis can, therefore, only be said to approximate to any simple diagrammatic presentation. A tribe is an exemplification of a segmentary tendency which is characteristic of the political structure as a whole. The reason why we speak of Nuer political groups, and of the tribe in particular, as relative groups and state that they are not easily described in terms of political morphology, is that political relations are dynamic. They are always changing in one direction or another. The most evident movement is towards fission. The tendency of tribes and tribal sections towards fission and internal opposition between their parts is balanced by a tendency in the direction of fusion, of the combination or amalgamation of groups. This tendency towards fusion is inherent in the segmentary character of Nuer political structure, for, although any group tends to split into opposed parts, these parts tend to fuse in relation to other groups. Hence fission and fusion are two aspects of the same segmentary principle and the Nuer tribe and its divisions are to be understood as a relation between these two contradictory, yet complementary, tendencies. Physical environment, way of livelihood, mode of distribution, poor communications, simple economy, &c., to some extent explain the incidence of political cleavage, but the tendency towards segmentation seems to be inherent in political structure itself.
777. Lineage System
Tribal unity cannot be accounted for by any of the facts we have so far mentioned, taken alone or in the aggregate, but only by reference to the lineage system. The Nuer clan is not an un-differentiated group of persons who recognize their common kinship, as are many African clans, but is highly segmented. The segments are genealogical structures, and we therefore refer to them as lineages and to the clan as an exogamous system of lineages which trace their descent to a common ancestor. The defining characteristic of a lineage is that the relationship of any member of it to other members can be exactly stated in genealogical terms. His relationship to members of other lineages of the same clan is, therefore, also known, since lineages are genealogically related. Thus, in the diagram below, A is a clan which is segmented into maximal lineages B and C and these again bifurcate