BRIEF [O.A.] Diederichs, Eugen Verfasser: Diederichs, Eugen Empfänger: Haeckel, Ernst Datierung: 11-04-1905
BRIEF [O.A.] Diederichs, Eugen Verfasser: Diederichs, Eugen Empfänger: Haeckel, Ernst Datierung: 19-03-1906