BRIEF [O.A.] Arnhold, Heinrich Author: Arnhold, Heinrich Recipient: Haeckel, Ernst Dating: 15.02.1904
BRIEF OHNE UMSCHLAG Bergh, Ernst van den Author: Bergh, Ernst van den Recipient: Haeckel, Ernst Dating: 06.08.1904
BRIEF [O.A.] Karuhtz, A... F... Author: Karuhtz, A... F... Recipient: Haeckel, Ernst Dating: 31.07.1904