BRIEF OHNE UMSCHLAG Geheeb, Adalbert Author: Geheeb, Adalbert Recipient: Haeckel, Ernst Dating: 02.05.1904
BRIEF [O.A.] Ziegler, Heinrich Ernst Author: Ziegler, Heinrich Ernst Recipient: Haeckel, Ernst Dating: 05.10.1904
POSTKARTE Wiedersheim, Robert Author: Wiedersheim, Robert Recipient: Haeckel, Ernst Dating: 15.02.1904