BRIEF [O.A.] Fleischmann, Albert Author: Fleischmann, Albert Recipient: Haeckel, Ernst Dating: 17.02.1892
BRIEF [O.A.] Falckenberg, Richard Author: Falckenberg, Richard Recipient: Haeckel, Ernst Dating: 15.02.1894
BRIEF [O.A.] Gerhardt, Dietrich Author: Gerhardt, Dietrich Recipient: Haeckel, Ernst Dating: 16.02.1904