BRIEF [O.A.] Schuster, Adolf Niklaus Author: Schuster, Adolf Niklaus Recipient: Haeckel, Ernst Dating: 23.10.1913
POSTKARTE Schuster, Adolf Niklaus Author: Schuster, Adolf Niklaus Recipient: Haeckel, Ernst Dating: 16.02.1913
BRIEF [O.A.] Knapp, Alfred Author: Knapp, Alfred Recipient: Haeckel, Ernst Corporation: Schweiz. Monistenbund, Ortsgruppe Zürich Dating: 24.12.1913
ANSICHTSKARTE Sprenger, Therese; Sprenger, Gustav Author: Sprenger, Therese; Sprenger, Gustav Recipient: Haeckel, Ernst Dating: 23.01.1913