BRIEF [O.A.] Casanova, Peregrín Author: Casanova, Peregrín Recipient: Haeckel, Ernst Dating: 22.03.1876
BRIEF [O.A.] Casanova, Peregrín Author: Casanova, Peregrín Recipient: Haeckel, Ernst Dating: 02.01.1876
BRIEF [O.A.] Casanova, Peregrín Author: Casanova, Peregrín Recipient: Haeckel, Ernst Dating: 21.01.1877
BRIEF [O.A.] Buen, Odon de Author: Buen, Odon de Recipient: Haeckel, Ernst Corporation: Comision Organizadora del viaje al Congreso Librepensador de Roma Dating: 16.07.1904