BRIEF [O.A.] Unbekannt Author: Unbekannt Recipient: Haeckel, Ernst Corporation: Wilhelm Engelmann Verlagsbuchhandlung Dating: 08.03.1905
BRIEF [O.A.] Weidenmüller. Joh... Author: Weidenmüller. Joh... Recipient: Haeckel, Ernst Corporation: Leipziger Freie Studentenschaft Dating: 23.06.1905
BRIEF [O.A.] Prokosch, E... Author: Prokosch, E... Recipient: Haeckel, Ernst Corporation: The University of Chicago Dating: 15.04.1905
BRIEF [O.A.] Prokosch, E... Author: Prokosch, E... Recipient: Haeckel, Ernst Corporation: The University of Chicago Dating: 07.04.1905
BRIEF [O.A.] Quelle Author: Quelle Recipient: Haeckel, Ernst Corporation: B. G. Teubner Buchhandlung Dating: 19.07.1905