ON high open downland in England, populations of the polymorphic landsnail Cepaea nemoralis tend not to vary with habitat, despite predation by at least one predator known to exert strong visual selection, the Song Thrush (Turdus ericetorum Turton). Morph frequencies remain relatively constant over areas of downiand very much larger than the panmictic area of populations of the species, and then change over long or short distances equally without reference to the nature of the background of the habitat (Cain and Currey, 1963a, b; Carter, 1968a). Cain and Currey (l963a) called these phenomena area effects, and suggested that the factors causing them override the effects of visual selection by predators. The latter form of selection almost certainly is the cause of populations resembling their backgrounds in the English lowland and wooded chalk hill regions investigated (Cain and Sheppard, 1954; Currey, Arnold and Carter, 1964; Arnold, 1966). Cain and Currey (1963a) concluded that selection is responsible for causing area effects for they cover huge areas and the populations are large. They suggested (1 963b) that if a sufficient number of area effects could be studied the nature of the non-visual, possibly climatic, selective agents causing different area effects might be elucidated. The results of a survey of populations on the eastern South Downs, chosen because the region is one of the sunniest in Britain, are described in this paper.