BRIEF [O.A.] Knack, Carl August Author: Knack, Carl August Recipient: Haeckel, Ernst Dating: 04.04.1907
BRIEF [O.A.] Bechhold Author: Bechhold Recipient: Haeckel, Ernst Corporation: Die Umschau Dating: 08.04.1907
BRIEF [O.A.] Pfungst, Arthur Author: Pfungst, Arthur Recipient: Haeckel, Ernst Corporation: Neuer Frankfurter Verlag Dating: 09.07.1907