BRIEF OHNE UMSCHLAG Plarre, Otto Author: Plarre, Otto Recipient: Haeckel, Ernst Corporation: Ortsgruppe Gera des Deutschen Monistenbundes Dating: 16.02.1919
BRIEF [O.A.] Plarre, Otto Author: Plarre, Otto Recipient: Haeckel, Ernst Corporation: D.M.B. Dating: 12.08.1919