Abstract.-When Darwin first proposed the possibility of sexual selection, he identified two mechanisms, male competition for mates and female choice of mates. Extending this classification, we distinguish two forms of mate choice, direct and indirect. This distinction clarifies the relationship between Darwin's two mechanisms and, furthermore, indicates that the potential scope for sexual selection is much wider than thus far realized. Direct mate choice, the focus of most research on sexual selection in recent decades, requires discrimination between attributes of individuals of the opposite sex. Indirect mate choice includes all other behavior or morphology that restricts an individual's set of potential mates. Possibilities for indirect mate choice include advertisement of fertility or copulation, evasive behavior, aggregation or synchronization with other individuals of the same sex, and preferences for mating in particular locations. In each of these cases, indirect mate choice sets the conditions for competition among individuals of the opposite sex and increases the chances of mating with a successful competitor. Like direct mate choice, indirect mate choice produces assortative mating. As a consequence, the genetic correlation between alleles affecting indirect choice and those affecting success in competition for mates can produce self-accelerating evolution of these complementary features of the sexes. The broad possibilities for indirect mate choice indicate that sexual selection has more pervasive influences on the coevolution of male and female characteristics than previously realized.Key words.-Darwin, male-male competition, mate choice, mating systems, sexual selection.Received September 18, 1995. Accepted September 21, 1995 Research has largely confirmed Darwin's (1859Darwin's ( , 1871 speculations about the influence of sexual selection on evolution. Although it required more than a century to settle many of the theoretical and empirical issues, most of the scope for sexual selection as described by Darwin is now widely accepted. Here we take the argument a step farther. By using recent clarifications of sexual selection and by introducing a new distinction, we show that sexual selection has ramifications for the evolution of sexual organisms that extend considerably beyond those originally imagined by Darwin and currently recognized.When introducing his theory of sexual selection, Darwin (1859) envisaged two distinct mechanisms. Sexual selection, he wrote, "depends ... on a struggle between the males for the possession of the females." As an alternative mechanism of sexual selection, he suggested that "female birds, by se-1 Corresponding Author. Present address: