BRIEF OHNE UMSCHLAG Haeckel, Ernst Author: Haeckel, Ernst Recipient: Strasburger, Eduard Dating: 09.12.1903
BRIEF OHNE UMSCHLAG Haeckel, Ernst Author: Haeckel, Ernst Recipient: de Gruyter, Walter Dating: 16.02.1904
BRIEF OHNE UMSCHLAG Haeckel, Ernst Author: Haeckel, Ernst Recipient: Engelmann, Wilhelm Dating: 16.02.1904
BILDPOSTKARTE Haeckel, Ernst Author: Haeckel, Ernst Recipient: Verworn, Max; Verworn, Josephine Dating: 29.12.1903