TELEGRAMM Recipient: Haeckel, Ernst Corporation: Redaktion des Wiesnik Znania in Petersburg Dating: 16.02.1914
BRIEF OHNE UMSCHLAG Henckel, Alexander Author: Henckel, Alexander Recipient: Haeckel, Ernst Dating: 26.01.1909
BRIEF OHNE UMSCHLAG Henckel, Alexander Author: Henckel, Alexander Recipient: Haeckel, Ernst Dating: 29.10.1913
BRIEF OHNE UMSCHLAG Henckel, Alexander Author: Henckel, Alexander Recipient: Haeckel, Ernst Corporation: Botanischer Garten der Universität St. Petersburg Dating: 09.11.1913