BRIEF [O.A.] Muller, H... C... Author: Muller, H... C... Recipient: Haeckel, Ernst Dating: 17.05.1883
BRIEF [O.A.] Hartoug Heijs van Zouteveen, H. Author: Hartoug Heijs van Zouteveen, H. Recipient: Haeckel, Ernst Dating: 28.04.1883