POSTKARTE Gyldén, Therese Author: Gyldén, Therese Recipient: Haeckel, Ernst; Haeckel, Agnes Dating: 30.12.1904
BRIEF OHNE UMSCHLAG Retzius, Gustaf Author: Retzius, Gustaf Recipient: Haeckel, Ernst Dating: 19.11.1895
BRIEF [O.A.] Lindhagen, D... G... Author: Lindhagen, D... G... Recipient: Haeckel, Ernst Corporation: Académie Royale des Sciences Dating: 06.01.1883