BRIEF [O.A.] Kretschmer, Ernst Author: Kretschmer, Ernst Recipient: Haeckel, Ernst Dating: 16.02.1911
BRIEF OHNE UMSCHLAG Plarre, Otto Author: Plarre, Otto Recipient: Haeckel, Ernst Corporation: Ortsgruppe Gera des Deutschen Monistenbundes Dating: 23.05.1911
BRIEF OHNE UMSCHLAG Plarre, Otto Author: Plarre, Otto Recipient: Haeckel, Ernst Corporation: Ortsgruppe Gera des Deutschen Monistenbundes Dating: 15.02.1911
BILDPOSTKARTE Haeckel, Emma Author: Haeckel, Emma Recipient: Haeckel, Ernst & Agnes [Eltern] Dating: 22.04.1911