POSTKARTE Bockelmann, Friedrich Author: Bockelmann, Friedrich Recipient: Haeckel, Ernst Dating: 16.02.1913
POSTKARTE Klinghammer, Waldemar Author: Klinghammer, Waldemar Recipient: Haeckel, Ernst Dating: 15.02.1913
BRIEF OHNE UMSCHLAG Dietrich, Rudolf Author: Dietrich, Rudolf Recipient: Haeckel, Ernst Dating: 09.11.1913