 German was his first language; that is, the German commonly spoken in Trieste in the 19th century.Most of his books were written in German.Recruited by Emílio Goeldi, Ducke began his work in Amazônia as an entomologist for the Museu Paraense Emílio Goeldi, but due to the influence of botanists Jacques Hüber and Paul Le Cointe, he switched to botany.