BRIEF OHNE UMSCHLAG Haeckel, Ernst Author: Haeckel, Ernst Recipient: Zirkel, Ferdinand Dating: 27.05.1872
BRIEF [O.A.] Greeff, Richard Author: Greeff, Richard Recipient: Haeckel, Ernst Corporation: Königliches Zoologisch Zootomisches Institut, Universität Marburg Dating: 12.09.1872