APA (7th ed.) Citation

Duckworth, W. L. H. (2013). Prehistoric Man. Project Gutenberg.

Chicago Style (17th ed.) Citation

Duckworth, W. L. H. Prehistoric Man. Project Gutenberg, 2013.

MLA (9th ed.) Citation

Duckworth, W. L. H. Prehistoric Man. Project Gutenberg, 2013.

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