BRIEF [O.A.] Böhtlingk, Arthur Author: Böhtlingk, Arthur Recipient: Haeckel, Ernst Dating: 07.10.1891
BRIEF [O.A.] Böhtlingk, Arthur Author: Böhtlingk, Arthur Recipient: Haeckel, Ernst Dating: 20.09.1916
BRIEF OHNE UMSCHLAG Bertheau, Georg Author: Bertheau, Georg Recipient: Haeckel, Ernst Dating: 15.02.1894
BRIEF OHNE UMSCHLAG Bertheau, Georg Author: Bertheau, Georg Recipient: Haeckel, Ernst Dating: 10.02.1904