Without analysis of the counter-factual, the list seems impressive. But had Howland died there is a high probability that there still would have been a 32nd, 41st, and 43rd US president; prominent 19th century poets; Alaskan governors; pediatricians and Hollywood leading men. And listing both Bush's seems gratuitous - why not list all the Baldwin brothers?<p>Probably the only two individuals on the list whose impact on US history might be considered irreproducible are Smith and to perhaps a lesser extent, Young because creating enduring spiritual systems [Mormonism] and political states [Utah] seems more dependent on individual characteristics than the roles filled by other individuals.