BRIEF [O.A.] Siebert, Friedrich Author: Siebert, Friedrich Recipient: Haeckel, Ernst Dating: 28.05.1906
BRIEF [O.A.] Siebert, Friedrich Author: Siebert, Friedrich Recipient: Haeckel, Ernst Dating: 12.07.1906
BRIEF OHNE UMSCHLAG Hertwig, Richard Author: Hertwig, Richard Recipient: Haeckel, Ernst Dating: 06.02.1906
BRIEF OHNE UMSCHLAG Hertwig, Richard Author: Hertwig, Richard Recipient: Haeckel, Ernst Dating: 01.11.1906
BRIEF [O.A.] Ludowici, Wilhelm Author: Ludowici, Wilhelm Recipient: Haeckel, Ernst Dating: 06.07.1906
BRIEF [O.A.] Molenaar, Heinrich Author: Molenaar, Heinrich Recipient: Haeckel, Ernst Dating: 11.10.1906