This article for the first time provides an edition and commentary of the letters of Friedrich Ratzel to his older colleague, teacher and mentor, Ernst Haeckel, which are kept in the archive of the Ernst-Haeckel-House (memorial museum) in Jena. Altogether fifteen letters and one postcard are presented. Haeckel's letters to Ratzel are considered to be lost. The edition is prefaced with a detailed description of Ratzel's life, career and work, as a part of the edition.