POSTKARTE Osborn, Henry Fairfield Author: Osborn, Henry Fairfield Recipient: Haeckel, Ernst Dating: ??.??.1906
BRIEF OHNE UMSCHLAG Magnussen, Harro Author: Magnussen, Harro Recipient: Haeckel, Ernst Dating: 12.02.1906
BRIEF OHNE UMSCHLAG Magnussen, Harro Author: Magnussen, Harro Recipient: Haeckel, Ernst Dating: 19.09.1906
BRIEF OHNE UMSCHLAG Magnussen, Harro Author: Magnussen, Harro Recipient: Haeckel, Ernst Dating: 09.12.1906
KARTE Bölsche, Wilhelm; Bölsche, Johanna Author: Bölsche, Wilhelm; Bölsche, Johanna Recipient: Haeckel, Ernst Dating: 24.12.1906
BRIEF OHNE UMSCHLAG Anonym ("H. B.") Author: Anonym ("H. B.") Recipient: Haeckel, Ernst Dating: 15.02.1906