BRIEF OHNE UMSCHLAG Fürbringer, Max Author: Fürbringer, Max Recipient: Haeckel, Ernst Dating: 26.11.1914
BRIEF [O.A.] Froelich, Heinrich Author: Froelich, Heinrich Recipient: Haeckel, Ernst Dating: 27.05.1914
BRIEF [O.A.] Franco, Isidoro de Author: Franco, Isidoro de Recipient: Haeckel, Ernst Dating: 28.02.1914