Over the course of the twentieth century, theoretical biology changed beyond all recognition. Although the field today is synonymous with mathematical biology, when it first emerged it had a drastically different agenda: to critically analyze the conceptual foundations of biology in order to resolve long-standing theoretical disputes, abstract from the ‘burden of details,’ and bring about the epistemic unification of biological science. The field began acquiring its now familiar mathematical character in the 1940s, as formal models became increasingly applied in different areas of biology, such as growth, ecology, genetics, and evolution. Regrettably, the early ‘philosophical’ period of theoretical biology has been almost completely forgotten and its existence is seldom acknowledged—let alone carefully examined (but see Nicholson & Gawne 2015, Baedke 2019). Much of this early discourse took place in a handful of book series, monographs, and journals, the majority of which were published in German (at least initially). Hence, it is perhaps not surprising that Anglophone scholars remain almost completely ignorant of this large, and surprisingly rich, body of literature.