In 1877, the young Alfred Espinas defended a philosophical study, ‘doctorat ès lettres’, at the Sorbonne University, entitled Des Sociétés animales. This was to become one of the principal sources of French organicist sociology. The paradox, however, is that this work seems to be fundamentally a study of natural science. Espinas tried to justify his position theoretically through two types of reciprocally exclusive and uncomplementary arguments. The first one consists in showing that only certain kinds of animal groupings belong legitimately, (...) if not academically, to sociology. The second one has a greater audacious and uncontrollable dimension. It consists in asserting that all pluricellular organisms are true societies. In this article we first focus on the role played by cellular theory in such a ‘sociologization’ of biology, concerning the extension of sociology. Second, we examine the importance of such confusion within the fields of extension, in aiming to explain the conceptual transfers between social sciences and life sciences, in the second half of the 19th century. (shrink)