BRIEF [O.A.] Recipient: Haeckel, Walter Corporation: The Rationalist Press Association Dating: 10.09.1919
BRIEF OHNE UMSCHLAG Gegenbaur, Fritz Author: Gegenbaur, Fritz Recipient: Haeckel, Walter Dating: 17.12.1912
BRIEF OHNE UMSCHLAG Arrhenius, Svante Author: Arrhenius, Svante Recipient: Haeckel, Walter Dating: 23.05.1920
BRIEF OHNE UMSCHLAG Ade, Mathilde Author: Ade, Mathilde Recipient: Haeckel, Walter Dating: 25.10.1903
BRIEF [O.A.] Unbekannt Author: Unbekannt Recipient: Haeckel, Walter Corporation: Wilhelm Engelmann Verlagsbuchhandlung Dating: 10.02.1921
BRIEF [O.A.] Dieckmann, Gertha Author: Dieckmann, Gertha Recipient: Haeckel, Walter Dating: 17.01.1923