Books, Brochures, and Chapters>Book:  Asimov, Isaac (2004-08-31), Foundation and Earth, Spectra, Retrieved on 2011-05-28

Periodicals>Journal Article:  Herrera, Ana (16-May-2011), Species are to ecosystems as cells are to the human body, according to a mathematical model, Carlos III University of Madrid , Retrieved on 2011-05-17

Source Material [www.eurekalert.org]

Books, Brochures, and Chapters>Book:  Lovelock, James (2000-11-23), Gaia: A New Look at Life on Earth, Oxford University Press, USA, Retrieved on 2011-04-11