Haeckel, E. The Riddle of the Universe at the close of the nineteenth century.
Chicago Style (17th ed.) CitationHaeckel, Ernst. The Riddle of the Universe at the Close of the Nineteenth Century.
MLA (9th ed.) CitationHaeckel, Ernst. The Riddle of the Universe at the Close of the Nineteenth Century.
Warning: These citations may not always be 100% accurate.