BRIEF [O.A.] Löbbecke, Otto von Author: Löbbecke, Otto von Recipient: Haeckel, Ernst Dating: 19.10.1914
BRIEF OHNE UMSCHLAG Leporin, Valerie Author: Leporin, Valerie Recipient: Haeckel, Ernst Dating: 04.11.1913
BRIEF [O.A.] Kohlrausch, Erich Author: Kohlrausch, Erich Recipient: Haeckel, Ernst Dating: 31.05.1917
BRIEF [O.A.] Kohlrausch, Erich Author: Kohlrausch, Erich Recipient: Haeckel, Ernst Dating: 04.06.1917