BRIEF [O.A.] Haecker, Valentin Author: Haecker, Valentin Recipient: Haeckel, Ernst Dating: 06.07.1904
BRIEF [O.A.] Haecker, Valentin Author: Haecker, Valentin Recipient: Haeckel, Ernst Dating: 11.07.1904
BRIEF OHNE UMSCHLAG Haecker, Valentin Author: Haecker, Valentin Recipient: Haeckel, Ernst Dating: 15.02.1909