BRIEF [O.A.] Abel-Musgrave, Curt Author: Abel-Musgrave, Curt Recipient: Haeckel, Ernst Dating: 01.05.1899
BRIEF OHNE UMSCHLAG Aegidi, Ludwig Karl Author: Aegidi, Ludwig Karl Recipient: Haeckel, Ernst Dating: 12.10.1852
BRIEF OHNE UMSCHLAG Anonym ("N. G.") Author: Anonym ("N. G.") Recipient: Haeckel, Ernst Dating: 19.06.1905