POSTKARTE Focke, Wilhelm Olbers Author: Focke, Wilhelm Olbers Recipient: Haeckel, Ernst Dating: 03.01.1918
BRIEF OHNE UMSCHLAG Focke, Wilhelm Olbers Author: Focke, Wilhelm Olbers Recipient: Haeckel, Ernst Dating: 14.02.1918
BRIEF OHNE UMSCHLAG Wilckens, Otto Author: Wilckens, Otto Recipient: Haeckel, Ernst Dating: 27.05.1918
BRIEF [O.A.] Lipsius, Friedrich Author: Lipsius, Friedrich Recipient: Haeckel, Ernst Dating: 15.02.1918