BRIEF [O.A.] Séckel, N... D... Author: Séckel, N... D... Recipient: Haeckel, Ernst Dating: 13.01.1911
BRIEF [O.A.] Jordan, David Starr Author: Jordan, David Starr Recipient: Haeckel, Ernst Corporation: Leland Stanford Junior University Dating: 19.06.1911
BRIEF OHNE UMSCHLAG Bender, Albert M. Author: Bender, Albert M. Recipient: Haeckel, Ernst Dating: 15.05.1911
BRIEF [O.A.] Bownes, Walter V... Author: Bownes, Walter V... Recipient: Haeckel, Ernst Corporation: The Ethic Association Dating: 30.01.1911