BRIEF [O.A.] Beneke-Schröer, A... Author: Beneke-Schröer, A... Recipient: Haeckel, Ernst Dating: 12.12.1909
BRIEF OHNE UMSCHLAG Benignus, Wilhelm Author: Benignus, Wilhelm Recipient: Haeckel, Ernst Dating: 31.05.1915
BRIEF OHNE UMSCHLAG Benignus, Wilhelm Author: Benignus, Wilhelm Recipient: Haeckel, Ernst Dating: 12.07.1915