BRIEF [O.A.] Gildemeister, E... Author: Gildemeister, E... Recipient: Haeckel, Ernst Dating: 15.01.1867
BRIEF [O.A.] Boettcher, Arthur Author: Boettcher, Arthur Recipient: Haeckel, Ernst Dating: 06.09.1867
BRIEF [O.A.] Kauffmann, Julius Author: Kauffmann, Julius Recipient: Haeckel, Ernst Dating: 03.07.1867