BRIEF OHNE UMSCHLAG Delbrück, Berthold Author: Delbrück, Berthold Recipient: Haeckel, Ernst Dating: 13.01.1898
BRIEF [O.A.] Camerano, Lorenzo Author: Camerano, Lorenzo Recipient: Haeckel, Ernst Dating: 14.09.1898
BRIEF OHNE UMSCHLAG Eggeling, Heinrich von Author: Eggeling, Heinrich von Recipient: Haeckel, Ernst Dating: 19.04.1898