Darwin came to realize that he could never devise an experiment that would demonstrate the development of new species by crossing hybrids. Instead, he turned to the problem of sterility within a species. The difference between Darwin's experimental and theoretical approach and that of French scientists working in the Muséum d'Histoire Naturelle in Paris can be shown most vividly by tracking Darwin's experiments in botany and his interest in those of Charles Naudin on hybrid plants. In his discussion of Mendel, Robert Olby has provided an interesting analysis of Naudin's hereditary concepts. I have approached some of the same work here with an emphasis on the question of sterility raised by Darwin, not simply between hybrids but within members of the same species and the light that he thought this could throw on both heredity and the origin of species.