John Muir (1997). “Nature Writings: The Story of My Boyhood and Youth, My First Summer in the Sierra, the Mountains of California, Stickeen, Selected Essays”, p.275, Library of America
"The Endangered Species Act: Preserving Wildlife, Wonder and Our Natural Heritage for 40 Years" by Jamie Rappaport Clark, www.huffingtonpost.com. January 2, 2013.