Darwin devoted much of his working life to the study of plant reproductive systems. He recognized that many of the intricacies of floral morphology had been shaped by natural selection in favour of outcrossing, and he clearly established the deleterious effects of self-fertilization on progeny. Although Darwin hypothesized the adaptive significance of self-fertilization under conditions of low mate availability, he held that a strategy of pure selfing would be strongly disadvantageous in the long term. Here, I briefly review these contributions to our understanding of plant reproduction. I then suggest that investigating two very different sexual systems, one in plants and the other in animals, would throw further light on the long-term implications of a commitment to reproduction exclusively by selfing.