BRIEF [O.A.] Unna, P...(aul) G...(erson) Author: Unna, P...(aul) G...(erson) Recipient: Haeckel, Ernst Dating: 24.02.1904
POSTKARTE Unna, P...(aul) G...(erson) Author: Unna, P...(aul) G...(erson) Recipient: Haeckel, Ernst Dating: 07.09.1904
BRIEF [O.A.] Gutmann, Herm... A... Author: Gutmann, Herm... A... Recipient: Haeckel, Ernst Dating: 12.02.1904