BRIEF OHNE UMSCHLAG Hartnack, Edmund Author: Hartnack, Edmund Recipient: Haeckel, Ernst Dating: 20.01.1857
BRIEF [O.A.] Dreyfus, N.N. Author: Dreyfus, N.N. Recipient: Haeckel, Ernst Corporation: Revue internationale de l'Ensignement, Armand Colin et Cie Dating: 09.10.188?
BRIEF OHNE UMSCHLAG Ritter, Paul von Author: Ritter, Paul von Recipient: Haeckel, Ernst Dating: 24.03.1894
BRIEF OHNE UMSCHLAG Ritter, Paul von Author: Ritter, Paul von Recipient: Haeckel, Ernst Dating: 29.03.1894