BRIEFABSCHRIFT Haeckel, Ernst Author: Haeckel, Ernst Recipient: Hellwald, Friedrich von Dating: 20.08.1873
BRIEFABSCHRIFT Haeckel, Ernst Author: Haeckel, Ernst Recipient: Hellwald, Friedrich von Dating: 24.11.1873
BRIEFABSCHRIFT Haeckel, Ernst Author: Haeckel, Ernst Recipient: Hellwald, Friedrich von Dating: 20.12.1873