BRIEF [O.A.] Leuchtenberg, Anna Author: Leuchtenberg, Anna Recipient: Haeckel, Ernst Dating: 14.02.1911
BRIEF [O.A.] Flechtheim, Alfred Author: Flechtheim, Alfred Recipient: Haeckel, Ernst Corporation: Galerie Alfred Flechtheim G.m.b.H. Dating: 17.02.1919