BRIEF [O.A.] Rehbaum, Theobald Author: Rehbaum, Theobald Recipient: Haeckel, Ernst Dating: 15.02.1904
BRIEF [O.A.] Leiningen-Westerburg, K. E. Graf von Author: Leiningen-Westerburg, K. E. Graf von Recipient: Haeckel, Ernst Dating: 07.05.1904
BRIEF [O.A.] Hoch, R. Author: Hoch, R. Recipient: Haeckel, Ernst Corporation: Freidenkerverein Wiesbaden Dating: 14.02.1904