BRIEF [O.A.] Holthusen, Ludwig Author: Holthusen, Ludwig Recipient: Haeckel, Ernst Corporation: Holthusen & Co. Dating: 22.05.1901
BRIEF [O.A.] Schultze, Leonhard Author: Schultze, Leonhard Recipient: Haeckel, Ernst Dating: 26.09.1901
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