BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Stregker & Schröder Verlagsbuchhandlung Dating: 11.02.1909
BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Stregker & Schröder Verlagsbuchhandlung Dating: 10.03.1909
BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Stregker & Schröder Verlagsbuchhandlung Dating: 03.12.1909