APA (7th ed.) Citation

Darwin, C. (2007). On the Origin of Species by Means of Natural Selection. Project Gutenberg.

Chicago Style (17th ed.) Citation

Darwin, Charles. On the Origin of Species by Means of Natural Selection. Project Gutenberg, 2007.

MLA (9th ed.) Citation

Darwin, Charles. On the Origin of Species by Means of Natural Selection. Project Gutenberg, 2007.

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