BRIEF OHNE UMSCHLAG Albrecht, Paul Author: Albrecht, Paul Recipient: Haeckel, Ernst Dating: 22.10.1875
BRIEF OHNE UMSCHLAG Otto, Johann Wilhelm (Ps. Otto Rutkowsky ?) Author: Otto, Johann Wilhelm (Ps. Otto Rutkowsky ?) Recipient: Haeckel, Ernst Dating: 18.10.1916
POSTKARTE Ziemssen, Siegfried Author: Ziemssen, Siegfried Recipient: Haeckel, Ernst Dating: 15.02.1904