Distylous flowering plant species are characterized by having two types of individuals that bear different forms of flowers: "pin" flowers with long styles and short stamens and "thrum" flowers with short styles and long stamens (Darwin, 1877; Frankel and Galun, 1977). In most cases this flower dimorphism is associated with a physiological self-incompatibility mechanism that prevents fertilization after self-pollination or pollen transfer between individuals of the same flower type, with the result that only pollination between forms results in fertilization (Frankel and Galun, 1977; de Nettancourt, 1977). In several angiosperm genera, distyly has evolved into dioecy (Baker, 1958, 1959; Bir Bahadur, 1968; Ornduff, 1966; Viulleumier, 1967; Opler et al., 1975), and in every case individuals bearing female flowers are evolutionarily derived from long-style individuals, while male plants are derived from the shortstyle form. The selective forces that may have brought about this transformation are not well understood (Lloyd, 1979). Here we propose a hypothesis to explain the evolution of distyly into dioecy.