POSTKARTE Darling, Ernest Willing Author: Darling, Ernest Willing Recipient: Haeckel, Ernst Dating: 28.07.1913
BRIEF OHNE UMSCHLAG Darling, Ernest Willing Author: Darling, Ernest Willing Recipient: Haeckel, Ernst Dating: 22.09.1913
BRIEF MIT UMSCHLAG Darling, Ernest Willing Author: Darling, Ernest Willing Recipient: Haeckel, Ernst Dating: 01.06.1914
UMSCHLAG Darling, Ernest Willing Author: Darling, Ernest Willing Recipient: Haeckel, Ernst Dating: 26.04.1914
BRIEF [O.A.] Rofoid, Charles Author: Rofoid, Charles Recipient: Haeckel, Ernst Corporation: University of California Dating: 11.12.1907