Between the mid-19th century and the mid-20th century, French biology, despite a handful of remarkable breakthroughs (e.g. Claude Bernard, Louis Pasteur), contributed only very marginally to the growth of biological thought. This has puzzled historians for decades, especially given the unbelievably strong opposition met by cell theory, evolutionary theory and genetics during that time in France. The aim of this paper is to show how a specific form of positivism was instrumental in shaping an epistemological attitude, shared by most scientists, which opposed any form of speculative theorization in biology. I show, first, that the French Society of Biology, which quickly became a highly influent institution, promoted exactly this kind of positivism, having already epitomized this position in its founding manifesto of 1849. Second, partly on the basis of secondary sources (Gley, 1899, Schnitter, 1992, Bange, 2009), I document the kind of research that was promoted within the French Soc...