BRIEF [O.A.] Unna, Paul Gerson Author: Unna, Paul Gerson Recipient: Haeckel, Ernst Dating: 24.02.1904
BRIEF [O.A.] Lüders, Ferdinand Author: Lüders, Ferdinand Recipient: Haeckel, Ernst Dating: 20.02.1904
BRIEF [O.A.] Münzet, R. Author: Münzet, R. Recipient: Haeckel, Ernst Corporation: Verein für Kunst und Wissenschaft Dating: 27.05.1904