The Bowler Hat, also known as The Derby Hat, was introduced in the 1850's. The Bowler hat is the first hat credited for being mass produced and priced to meet everyone's budget, because of this the Derby Hats were worn by virtually every class of society. It was so popular that it replaced the Top Hat in the late nineteenth century as the most popular hat for men. The Derby remained the preferred men's hat for almost seventy years until the 1920's when The Fedora eclipsed The Derby to became the most popular hat style.
The derby hat comes in many different styles from the low-crown to the stingy brim. The bowler started out as a protective hat for coachmen and gamekeepers. They were stiffened and protected against branches and other obstacles, while riding on horses. Eventually the derby style caught on and became fashionable. The term Bowler is an English name for the hat. The Americans started calling the hat a derby when the Earl of Derby wore the hat at the Derby races.
Wikipedia: Bowler Hat