 The question of biological heredity emerged in the nineteenth century in diverse fields of practice and gave rise to various theories, which often aimed to provide a unified view of heredity, development, and evolution (Müller-Wille &amp; Rheinberger 2012: Chs. 2–5). Gregor Mendel (1822–1884) did not aim at a unifying theory, nor did he set out to discover laws governing heredity. Historians have argued, instead, that he was concerned with specific questions regarding the role of hybridization in evolution (Olby 1979; cf. Müller-Wille 2018). Other hybridists had crossed plants of different species and varieties, studied their offspring, and asked whether new species could emerge in this process. Issues regarding the classification of hybrids emerged: how did hybrids resulting from crosses between pure species differ from offspring of parent organisms that were hybrids themselves; and how could their characteristics be interpreted with respect to those of their parents when it came to determining their type? Mendel (1866 [2016]) aimed to address such questions in his experiments with garden peas and in this respect was indeed indebted to his predecessors. Nonetheless, he introduced several changes in the experimental system employed by the hybridists that allowed him to arrive at more specific results regarding the distribution of types in several generations of offspring (Müller-Wille &amp; Orel 2007). First, he concentrated on varieties that differed only in one or a few characteristics. This enabled him to move from classifying the offspring according to type to a focus on the distribution of traits that characterized the types. Second, he made sure that he obtained offspring in numbers that allowed for a statistical treatment of the distribution of characters. In this way, he was able to observe regularities in the appearance of types in the third generation (plants resulting from crosses between hybrids) that previous hybridists where not able to detect. Finally, he developed a notation system where letters represented types of germ cells, which he used to record his results and compare them with theoretical predictions. These innovations enabled Mendel to arrive at a set of regularities regarding