BRIEF [O.A.] Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Dating: 16.10.1915
BRIEF OHNE UMSCHLAG Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Corporation: Verlagsbuchhandlung Georg Reimer in Berlin Dating: 31.01.1916
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: 1916.??.?? [?]
BRIEF OHNE UMSCHLAG Gruyter, Walter de Author: Gruyter, Walter de Recipient: Haeckel, Ernst Corporation: Verlagsbuchhandlung Georg Reimer in Berlin Dating: 07.08.1918
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