Elbert Hubbard (2009). “Love, Life & Work: Being a Book of Opinions Reasonably Good-Natured Concerning How to Attain the Highest Happiness for One's Self with the Least Possible Harm to Others”, p.65, The Floating Press
Elbert Hubbard (1905). “Little Journeys to Homes of Great Scientists ...: Copernicus. Galileo. Sir Isaac Newton. Humboldt. Sir William Herschel. Charles Darwin”