BRIEF [O.A.] Langenbruch, W... Author: Langenbruch, W... Recipient: Haeckel, Ernst Dating: 01.08.1917
BRIEF [O.A.] Kempner-Hochstädt, Max Author: Kempner-Hochstädt, Max Recipient: Haeckel, Ernst Dating: 22.11.1914
BRIEF [O.A.] Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 16.10.1915
BRIEF [O.A.] Hoensbroech, Graf Paul von Author: Hoensbroech, Graf Paul von Recipient: Haeckel, Ernst Dating: 24.04.1915
BRIEF [O.A.] Hoensbroech, Graf Paul von Author: Hoensbroech, Graf Paul von Recipient: Haeckel, Ernst Dating: 11.10.1916