BRIEF [O.A.] Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 13.08.1919
BRIEFABSCHRIFT Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 28.04.1919
BRIEFABSCHRIFT Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 02.06.1919
BRIEFABSCHRIFT Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 05.06.1919
BRIEFABSCHRIFT Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 24.06.1919
BRIEFABSCHRIFT Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 28.07.1919