BRIEF [O.A.] Harting, (Pieter) Paul [?] Author: Harting, (Pieter) Paul [?] Recipient: Haeckel, Ernst Dating: 07.07.1868
BRIEF [O.A.] Harting, (Pieter) Paul [?] Author: Harting, (Pieter) Paul [?] Recipient: Haeckel, Ernst Dating: 11.01.1869
BRIEF [O.A.] Harting, (Pieter) Paul [?] Author: Harting, (Pieter) Paul [?] Recipient: Haeckel, Ernst Dating: 26.06.1869