 In Layard's 1867 treatise on the regional avifauna, he arbitrarily defined "South Africa" as the region south of 28° South.Sharpe's 1884 revision of Layard's work extended the boundary to the Cuanza and Zambezi rivers, believing that the latter is a natural avifaunal limit.Stark and Sclater, possibly influenced by national boundaries determined at the Berlin Conference, substituted the Cuanza with the Kunene River for the first of their volumes, which appeared in 1900.