BRIEF [O.A.] Recipient: Haeckel, Ernst Corporation: Kaufmännischer Verein Frankfurt a. M. Dating: 24.08.1875
BRIEF [O.A.] Hartmann Author: Hartmann Recipient: Haeckel, Ernst Corporation: Museums-Gesellschaft zu Frankfurt a. M. Dating: 05.08.1875
BRIEF [O.A.] Hartmann Author: Hartmann Recipient: Haeckel, Ernst Corporation: Museums-Gesellschaft zu Frankfurt a. M. Dating: 14.08.1875