BRIEF OHNE UMSCHLAG Haeckel, Ernst Author: Haeckel, Ernst Recipient: Lachmann, Johannes Dating: 15.02.1857
BRIEFENTWURF Haeckel, Ernst Author: Haeckel, Ernst Recipient: Hudtwalker, Martin Hieronymus Dating: ??.08.1860
WIDMUNG Haeckel, Ernst Author: Haeckel, Ernst Recipient: Sarasin, Paul; Sarasin, Fritz Dating: 1906.??.?? [?]