BRIEF [O.A.] Holthusen, Ludwig Author: Holthusen, Ludwig Recipient: Haeckel, Ernst Corporation: Holthusen & Co. Dating: 22.05.1901
BRIEF OHNE UMSCHLAG Watts, Charles A. Author: Watts, Charles A. Recipient: Haeckel, Ernst Corporation: The Rationalist Press Association Dating: 10.09.1901
BRIEF OHNE UMSCHLAG Schmitz, L. Dora Author: Schmitz, L. Dora Recipient: Haeckel, Ernst Dating: 12.02.1901
BRIEF OHNE UMSCHLAG Bernard, Henry Author: Bernard, Henry Recipient: Haeckel, Ernst Corporation: National History Museum Dating: 08.11.1901
BRIEF [O.A.] Jagan, C... E... Author: Jagan, C... E... Recipient: Haeckel, Ernst Corporation: British Museum (Natural History) Dating: 21.03.1901