BRIEF [O.A.] Nippold, W. K. A. Author: Nippold, W. K. A. Recipient: Haeckel, Ernst Dating: 14.10.1903
BRIEF [O.A.] Nippold, W. K. A. Author: Nippold, W. K. A. Recipient: Haeckel, Ernst Dating: 23.10.1903
BRIEF [O.A.] Reinhard, W. Author: Reinhard, W. Recipient: Haeckel, Ernst Corporation: Redaktion des Pestalozzikalenders Dating: 05.12.1914
BRIEF OHNE UMSCHLAG Studer, Theophil Author: Studer, Theophil Recipient: Haeckel, Ernst Corporation: Internationaler Zoologen-Kongress Dating: 13.04.1904
BRIEF [O.A.] N.N. Author: N.N. Recipient: Haeckel, Ernst Corporation: Europäische Gesellschaft zur Verständigung der Intellektuellen, Bern Dating: ??.05.1915