BRIEF [O.A.] Engelsmann, Elisabeth Author: Engelsmann, Elisabeth Recipient: Haeckel, Ernst Dating: 15.02.1913
BRIEF [O.A.] Engelsmann, Elisabeth Author: Engelsmann, Elisabeth Recipient: Haeckel, Ernst Dating: 30.12.1913
BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Deutscher Monistenbund, Ortsgruppe Stuttgart Dating: 17.02.1913
BRIEF [O.A.] Keller, Walther Author: Keller, Walther Recipient: Haeckel, Ernst Corporation: Franckh'sche Verlagshandlung Dating: 08.07.1913