APA (7th ed.) Citation

Darwin, C. (2005). On the Origin of Species. Project Gutenberg.

Chicago Style (17th ed.) Citation

Darwin, Charles. On the Origin of Species. Project Gutenberg, 2005.

MLA (9th ed.) Citation

Darwin, Charles. On the Origin of Species. Project Gutenberg, 2005.

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