BRIEF [O.A.] Engelsmann, Elisabeth Author: Engelsmann, Elisabeth Recipient: Haeckel, Ernst Dating: 30.12.1911
BRIEF [O.A.] Regensberg, Friedr... Author: Regensberg, Friedr... Recipient: Haeckel, Ernst Corporation: Verein Naturschutzpark Dating: 13.02.1911
BRIEF [O.A.] Regensberg, Friedr... Author: Regensberg, Friedr... Recipient: Haeckel, Ernst Corporation: Verein Naturschutzpark Dating: 27.02.1911
BRIEF [O.A.] Georgi, Albert Author: Georgi, Albert Recipient: Haeckel, Ernst Corporation: Deutscher Monistenbund, Ortsgruppe Stuttgart-Cannstatt Dating: 15.02.1911