BRIEF [O.A.] Frenkel, Ferdinand Author: Frenkel, Ferdinand Recipient: Haeckel, Ernst Dating: 11.03.1919
BRIEF [O.A.] Geffcken, Heinrich Author: Geffcken, Heinrich Recipient: Haeckel, Ernst Dating: ??.06.1919
BRIEF [O.A.] Geffcken, Heinrich Author: Geffcken, Heinrich Recipient: Haeckel, Ernst Dating: 25.07.1919
BRIEF [O.A.] Fleischhauer, Fritz Author: Fleischhauer, Fritz Recipient: Haeckel, Ernst Dating: 26.02.1919
BRIEF [O.A.] Fleischhauer, Fritz Author: Fleischhauer, Fritz Recipient: Haeckel, Ernst Dating: 07.04.1919