BRIEF [O.A.] Jacobi-Scherbening, S... Author: Jacobi-Scherbening, S... Recipient: Haeckel, Ernst Dating: 24.04.1905
BRIEF [O.A.] Jericho, Ferdinand Author: Jericho, Ferdinand Recipient: Haeckel, Ernst Corporation: Deutscher Monistenbund, Ortsgruppe Magdeburg Dating: 15.04.1910
BRIEF [O.A.] Strauß-Hauswaldt, Margarete Author: Strauß-Hauswaldt, Margarete Recipient: Haeckel, Ernst