BRIEF OHNE UMSCHLAG Hertwig, Richard Author: Hertwig, Richard Recipient: Haeckel, Ernst Dating: 06.03.1907
BRIEF OHNE UMSCHLAG Hertwig, Richard Author: Hertwig, Richard Recipient: Haeckel, Ernst Dating: 30.03.1907
BRIEFABSCHRIFT Hertwig, Richard Author: Hertwig, Richard Recipient: Haeckel, Ernst Dating: 22.08.1907