BRIEF [O.A.] Walther, Johannes Author: Walther, Johannes Recipient: Haeckel, Ernst Dating: 27.07.1897
BRIEF [O.A.] Froelich, Heinrich Author: Froelich, Heinrich Recipient: Haeckel, Ernst Dating: 23.02.1913
BRIEF [O.A.] Froelich, Heinrich Author: Froelich, Heinrich Recipient: Haeckel, Ernst Dating: 27.05.1914