• Ernst Haeckel, The Riddle of the Universe (Die Welträthsel or Die Weltraetsel, 1895–1899), Publisher: Prometheus Books, Buffalo, NY, 1992, reprint edition, paperback, 405 pages, illustrated, ISBN 0-87975-746-9.
  • Ernst Haeckel, Monism as Connecting Religion and Science ("translated from German by J. Gilchrist, M.A., B.Sc., PH.D."), Project Gutenberg Literary Archive Foundation, webpage: GutenbergOrg-7mono10 (for free download).