Morgan, Thomas Hunt; Alfred H. Sturtevant, H. J. Muller and C. B. Bridges (1915). The Mechanism of Mendelian Heredity. New York: Henry Holt.{{cite book}}: CS1 maint: multiple names: authors list (link)
"Morgan's passion for experimentation was symptomatic of his general skepticism and his distaste for speculation. He believed only what could be proven. He was said to be an atheist, and I have always believed that he was. Everything I knew about him—his skepticism, his honesty—was consistent with disbelief in the supernatural." Norman H. Horowitz, T. H. Morgan at Caltech: A Reminiscence, Genetics, Vol. 149, 1629–1632, August 1998.