BRIEF OHNE UMSCHLAG Kükenthal, Willy Author: Kükenthal, Willy Recipient: Haeckel, Ernst Dating: 02.11.1898
TELEGRAMM Gerhardt, Ulrich [et al.] Author: Gerhardt, Ulrich [et al.] Recipient: Haeckel, Ernst Dating: 16.02.1914
BRIEF OHNE UMSCHLAG Floerke, Wilhelm (Willy) Author: Floerke, Wilhelm (Willy) Recipient: Haeckel, Ernst Dating: 16.02.1909