Darwin probably had never heard of Mendel. Nor had anyone but Nägeli—until 1881, when Die Pflanzen-Mischlinge by Wilhelm Olbers Focke appeared. The book, known familiarly as Focke, summarized the world’s leading plant experiments and cited Mendel’s work fifteen times. “Mendel believed that he found constant numerical proportions between the types of hybrids,” Focke wrote, adding that the monk’s work followed the tradition of early hybridizers. Like them, Mendel found that hybrids tend to revert to parental form and that old, apparently lost characteristics can reemerge generations later.