APA (7th ed.) Citation

Darwin, C. (2011, January). Variation under Nature. The Origin of Species, 120.

Chicago Style (17th ed.) Citation

Darwin, Charles. "Variation Under Nature." The Origin of Species Jan. 2011: 120.

MLA (9th ed.) Citation

Darwin, Charles. "Variation Under Nature." The Origin of Species, Jan. 2011, p. 120.

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