BRIEF [O.A.] Birkhahn, Robert C... Author: Birkhahn, Robert C... Recipient: Haeckel, Ernst Dating: 13.01.1904
BRIEF [O.A.] Rusby, George L... Author: Rusby, George L... Recipient: Haeckel, Ernst Dating: 23.08.1904
BRIEF [O.A.] Lilienthal, Friedrich W... Author: Lilienthal, Friedrich W... Recipient: Haeckel, Ernst Dating: 02.02.1904
BRIEF [O.A.] Ward, Lester F... Author: Ward, Lester F... Recipient: Haeckel, Ernst Dating: 11.11.1904