Morphogenesis and differentiation are the words which are used today to indicate the two basic modalities of ontogenesis in biology. Morphogenesis refers to the process of progressive ‘complexification’ during embryonic development, whereas differentiation refers to that of the functional specialization of the egg’s cells. Paradoxically, for more than a century, the term of differentiation has been employed to express the first of these variables, mainly if not exclusively. How did it come in developmental biology to name the physiological phenomenon which is parallel to structural complexification and which 19th century naturalists readily called the division of physiological labour? In fact, it appears that this inversion is only the latest episode in a series of semantic adventures affecting the history of the concept over nearly two centuries. We intend to explain them in this communication. We shall retrace the following steps: the term ‘differentiation’ appears in the field of animal anatomy at the beginning of the 19th century, because of its resemblance to ‘complication’, which is currently used by morphologists at this period (term which is itself practically synonymous with what 18th century naturalists called the “composition of the organization”). From comparative anatomy the term will migrate to embryology when the principle of epigenesis triumphs, and will know a considerable rise in epistemological status, reaching its climax when ranked by Von Baer as a fundamental concept of new scientific embryology. Its pairing with the concept of the division of physiological labour will confer on differentiation the role of criterion with which anatomists on the one hand, embryologists on the other hand, will judge the degree of improvement reached by embryonic formations and adult forms, respectively. Then the morphological significance of the term is enriched with a new evolutionary meaning, through the diffusion of the Darwinian theory and the adoption of the biogenetic law. At this degree of conceptual elaboration, we witness an extension of differentiation’s field to the phenomena concerned with anthropology (comparative analysis of different societies) and history (comparative analysis of different formations of the same society) thanks to sociologists such as Spencer who adopted the principle of cultural evolutionism. Evolutionary meanings of differentiation will regress correlatively, in life sciences and social sciences, during the inter-war period, to such an extent that its legitimate field of extension is reduced only to developmental biology. Consequently with the invalidation of the problem of the organic basis of living beings, differentiation loses its quasi etiological function (degree of differentiation as criterion of organic improvement) and comes back to its modal and descriptive primitive status. We shall focus in this paper a part of the history of this complex and interdisciplinary concept of differentiation, of which contemporary social sciences inherit to some extent, showing particularly the benefits of such an inquiry for the understanding of cultural evolutionism, which was so influent in the past social sciences, and even today.