We tried to point out in this paper the contribution that had the scientist Léon de Rosny, president of the Society of Ethnography (1859, Paris) Orientalist and Americanist, author of a work of epistemological reflection, with impressive field researches, to the study of the "Danube Populations". Our investigation exploits information from the archives and his correspondence with the Romanian historian and politician V. A. Urechia kept in the Library of the Romanian Academy and in “V.A. Urechia" Library of Galati, from the press and the scientific publications of the time, from Romania and abroad, memoirs, etc. We highlight also the amplitude of this project, the “challenge worthy of Léon de Rosny's appetite” and the role of the encounters and of the intellectual networks of the time for the structure and the profoundness of the approaches on this topic.