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"LifeScience.com" N.p., n.d. Web. 30 Sept. 2013.
"Wikipedia." Wikipedia. N.p., n.d. Web. 30 Sept. 2013
"Frontpage: Center Callouts." Grand Canyon National Geographic Visitor Center. N.p., n.d. Web. 30 Sept. 2013.
"History.com." History.com. A&E Television Networks, n.d. Web. 30 Sept. 2013.
"LifeScience.com" N.p., n.d. Web. 30 Sept. 2013.
"Wikipedia." Wikipedia. N.p., n.d. Web. 30 Sept. 2013