 On their part, geneticists became increasingly interested in the chemical nature of genes and their roles in metabolism and development. Several researchers had emphasized analogies between gene action and enzyme mediated chemical reactions (Ravin 1977). Using their terminology, Hermann Muller (1890–1967) and others observed that the gene had two characteristics that needed to be elucidated. First, it was autocatalytic, i.e., it was capable of reproducing itself for cell division and heredity to happen; second, it was heterocatalytic, i.e., it was able to facilitate the production of other chemical materials to perform its function (Muller 1922, 1947). What was peculiar regarding the first aspect was that genes were still capable of autocatalysis when they underwent mutational change, and they faithfully reproduced the alteration. Hence, for Muller, mutation was the key to the material nature of the gene. In the late 1920s, he and others were able to establish the mutagenic agency of radiation (Muller 1927; see Campos 2015: Chs. 5–6). The fact that genes could be “hit” and changed through X-rays supported the idea that they were individual entities that could be physically and chemically characterized.