BRIEF [O.A.] Arnhold, Heinrich Author: Arnhold, Heinrich Recipient: Haeckel, Ernst Dating: 15.02.1904
BRIEF OHNE UMSCHLAG Bergh, Ernst van den Author: Bergh, Ernst van den Recipient: Haeckel, Ernst Dating: 06.08.1904
BRIEF OHNE UMSCHLAG Giltsch, Adolf Author: Giltsch, Adolf Recipient: Haeckel, Ernst Dating: 02.03.1904
BRIEF OHNE UMSCHLAG Hooper, Charles E. Author: Hooper, Charles E. Recipient: Haeckel, Ernst Corporation: The Rationalist Press Association Dating: 19.12.1904