BRIEF [O.A.] Ankenbrand, Ludwig Author: Ankenbrand, Ludwig Recipient: Haeckel, Ernst Dating: 12.08.1919
POSTKARTE Keitenz, Rudolph [?] Author: Keitenz, Rudolph [?] Recipient: Haeckel, Ernst Dating: 22.12.????
BRIEF OHNE UMSCHLAG Katzenstein, Herbert Author: Katzenstein, Herbert Recipient: Haeckel, Ernst Dating: 13.02.1904