BRIEF OHNE UMSCHLAG Haeckel, Ernst Author: Haeckel, Ernst Recipient: Haeckel, Karl Dating: 14.10.1890
BRIEF OHNE UMSCHLAG Mulder, Lodewijk Author: Mulder, Lodewijk Recipient: Haeckel, Ernst Dating: 15.02.1894
BRIEF OHNE UMSCHLAG Mulder, J. A., de Villeneuve Author: Mulder, J. A., de Villeneuve Recipient: Haeckel, Ernst Dating: 14.05.1907