BRIEF [O.A.] Leemann, J... A... Author: Leemann, J... A... Recipient: Haeckel, Ernst Dating: 20.09.1912
BRIEF [O.A.] Leemann, J... A... Author: Leemann, J... A... Recipient: Haeckel, Ernst Dating: 10.05.1913
BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Deutscher Monistenbund, Ortsgruppe Hannover Dating: 13.03.1919