BRIEF [O.A.] Kirchhoff, Alfred Author: Kirchhoff, Alfred Recipient: Haeckel, Ernst Dating: 14.02.1891
BRIEF [O.A.] Kirchhoff, Alfred Author: Kirchhoff, Alfred Recipient: Haeckel, Ernst Dating: 06.11.1893
BRIEF [O.A.] Kirchhoff, Alfred Author: Kirchhoff, Alfred Recipient: Haeckel, Ernst Dating: 09.03.1899