The Rockefellers are associated with many things, among them unfettered wealth and equally generous philanthropy. But missing from the long list of Rockefeller accomplishments is, in a word: Bullmastiffs. The connection between America's richest family and Britain's premier estate-guardian breed is little known. But in the 1930s and '40s - very discreetly and very quietly, as was their wont - the Rockefellers imported some of the nation's earliest...