BRIEF [O.A.] Flothuis, Marius Hendrikus Author: Flothuis, Marius Hendrikus Recipient: Haeckel, Walter Dating: 18.08.1919
BRIEF [O.A.] Kuyper, H... A... Author: Kuyper, H... A... Recipient: Haeckel, Ernst Dating: 01.08.1904
BRIEF [O.A.] Katz, J... R... Author: Katz, J... R... Recipient: Haeckel, Ernst Corporation: Société Sections pour l`Oeuvre Scientifique Dating: 01.02.1905
BRIEF [O.A.] Kreunen, H... M... Author: Kreunen, H... M... Recipient: Haeckel, Ernst Dating: 12.03.1907