BRIEF [O.A.] Aschenberg, Alex... Author: Aschenberg, Alex... Recipient: Haeckel, Ernst Dating: 10.01.1893
BRIEF [O.A.] Brandt-Caspari, Alfred Author: Brandt-Caspari, Alfred Recipient: Haeckel, Ernst Dating: 22.12.1911
BRIEF OHNE UMSCHLAG Franck, Alex Author: Franck, Alex Recipient: Haeckel, Ernst Corporation: Lithographische Kunstanstalt, Heinrich Franck, Inh. Alex Franck Dating: 29.08.1903