APA (7th ed.) Citation

Haeckel, E., & McCabe, J. (2013). The Riddle of the Universe at the close of the nineteenth century. Project Gutenberg.

Chicago Style (17th ed.) Citation

Haeckel, Ernst, and Joseph McCabe. The Riddle of the Universe at the Close of the Nineteenth Century. Project Gutenberg, 2013.

MLA (9th ed.) Citation

Haeckel, Ernst, and Joseph McCabe. The Riddle of the Universe at the Close of the Nineteenth Century. Project Gutenberg, 2013.

Warning: These citations may not always be 100% accurate.