BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Stregker & Schröder Verlagsbuchhandlung Dating: 11.02.1909
BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Stregker & Schröder Verlagsbuchhandlung Dating: 10.03.1909
BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Stregker & Schröder Verlagsbuchhandlung Dating: 03.12.1909
BRIEF [O.A.] Engelhardt, Ewald Author: Engelhardt, Ewald Recipient: Haeckel, Ernst Dating: ??.11.1909
BRIEF [O.A.] Gmelin, Johann Georg Author: Gmelin, Johann Georg Recipient: Haeckel, Ernst Dating: 05.09.1909