BRIEF [O.A.] Neustadtel, Arthur Author: Neustadtel, Arthur Recipient: Haeckel, Ernst Dating: 04.07.1903
BRIEF [O.A.] Kreutzer, Erwin Rud... Author: Kreutzer, Erwin Rud... Recipient: Haeckel, Ernst Dating: 07.03.1904
BRIEF [O.A.] Nestel Eichhausen, Gustav von Author: Nestel Eichhausen, Gustav von Recipient: Haeckel, Ernst Dating: 16.08.1914