BRIEF [O.A.] Schaefer-Gerdau, Käte Author: Schaefer-Gerdau, Käte Recipient: Haeckel, Ernst Dating: 17.06.1914
BRIEF [O.A.] Schaefer-Gerdau, Käte Author: Schaefer-Gerdau, Käte Recipient: Haeckel, Ernst Dating: 21.06.1914
BRIEF [O.A.] Schaefer-Gerdau, Käte Author: Schaefer-Gerdau, Käte Recipient: Haeckel, Ernst Dating: 15.05.1918
BRIEF [O.A.] Schaefer-Gerdau, Käte Author: Schaefer-Gerdau, Käte Recipient: Haeckel, Ernst Dating: 26.05.1918