[en] The Third Chimpanzee for Young People Triangle Square
[en] [Gutenberg 2010] • The Autobiography of Charles Darwin W.W. Norton & Company
[de] [Gutenberg 50081] • Natur und Mensch / Sechs Abschnitte aus Werken von Ernst Haeckel Forgotten Books
[en] [Gutenberg 36520] • The Descent of Man and Selection in Relation to Sex, Vol. II (1st Edition) Createspace Independent Publishing Platform