BRIEF OHNE UMSCHLAG Delbrück, Berthold Author: Delbrück, Berthold Recipient: Haeckel, Ernst Dating: 13.02.1891
BRIEF OHNE UMSCHLAG Delbrück, Berthold Author: Delbrück, Berthold Recipient: Haeckel, Ernst Dating: 20.05.1895
BRIEF OHNE UMSCHLAG Delbrück, Berthold Author: Delbrück, Berthold Recipient: Haeckel, Ernst Dating: 13.01.1898
BRIEF [O.A.] Diederichs, Eugen Author: Diederichs, Eugen Recipient: Haeckel, Ernst Corporation: Eugen Diederichs Verlag Dating: 26.03.1905
BRIEF OHNE UMSCHLAG Engelmann, Theodor Wilhelm Author: Engelmann, Theodor Wilhelm Recipient: Haeckel, Ernst Dating: 16.07.??