POSTKARTE Osborn, Henry Fairfield Author: Osborn, Henry Fairfield Recipient: Haeckel, Ernst Dating: ??.??.1906
BRIEF OHNE UMSCHLAG Becker, Franz Johann Jakob Emil Author: Becker, Franz Johann Jakob Emil Recipient: Haeckel, Ernst Dating: 26.09.1906
BRIEF OHNE UMSCHLAG Becker, Daniel Georg Author: Becker, Daniel Georg Recipient: Haeckel, Ernst Dating: 25.11.1906