 Marcel Weber (2014) notes that, because many areas of modern biology “are profoundly experimental disciplines, an increased attention [in philosophy of biology] to the experiment in biology was inevitable” (see also entry on  gene).  Yet, how an area of biology becomes experimental in the first place as well as the implications of that shift also warrant attention. A recounting of Johannsen (1911) in this vein serves not only to introduce his original genotype-phenotype distinction, but also to point to various conceptual and methodological complexities that are associated with redefining heredity on the basis of breeding experiments. The detail in this section and the next is important not as a matter of doing history, but to insert distance from the predominant current-day meanings and establish a space in which modern users of his terms can consider issues that had been left behind.