BRIEF [O.A.] Fenk, Willi Author: Fenk, Willi Recipient: Haeckel, Ernst Corporation: Deutscher Monistenbund, Ortsgruppe Erfurt Dating: 15.02.1913
BRIEF [O.A.] Retowski, Alexander Author: Retowski, Alexander Recipient: Haeckel, Ernst Dating: 31.01.1905
