BRIEF [O.A.] Haeckel, Siegfried Author: Haeckel, Siegfried Recipient: Haeckel, Ernst Dating: 25.01.1902
BRIEF [O.A.] Haeckel, Siegfried Author: Haeckel, Siegfried Recipient: Haeckel, Ernst Dating: 20.01.1902
BRIEF [O.A.] Haeckel, Siegfried Author: Haeckel, Siegfried Recipient: Haeckel, Ernst Dating: 02.12.1901
BRIEF [O.A.] Haeckel, Siegfried Author: Haeckel, Siegfried Recipient: Haeckel, Ernst Dating: 08.12.1901
BRIEF [O.A.] Haeckel, Siegfried Author: Haeckel, Siegfried Recipient: Haeckel, Ernst Dating: 16.05.????