In their attempt to explain the social stability of African "primitive" stateless societies where they undertook their fieldwork, Evans-Pritchard (1940) and Meyer Fortes (1945) argued that the Tallensi and the Nuer were primarily organized around unilineal descent groups. Such groups are characterized by common purposes, such as administering property or defending against attacks; they form a permanent social structure that persists well beyond the lifespan of their members. In the case of the Tallensi and the Nuer, these corporate groups were based on kinship which in turn fitted into the larger structures of unilineal descent; consequently Evans-Pritchard's and Fortes' model is called "descent theory". Moreover, in this African context territorial divisions were aligned with lineages; descent theory therefore synthesized both blood and soil as the same. Affinal ties with the parent through whom descent is not reckoned, however, are considered to be merely complementary or secondary (Fortes created the concept of "complementary filiation"), with the reckoning of kinship through descent being considered the primary organizing force of social systems. Because of its strong emphasis on unilineal descent, this new kinship theory came to be called "descent theory".