[size=13pt]Like weeds, NFL's challengers rise, flourish and fade[/size]

But some leave lasting marks

By Barry Osborne and Mike Judson
Denver Post Staff Writers

Article Last Updated: 09/05/2007 06:20:50 PM MDT

Since its formation in 1920, the National Football League (known as the American Professional Football Association in 1920 and 1921) has had several start-up leagues challenge its supremacy as the sport's premier league. Here are a few of the contenders and pretenders that were wiped out or absorbed by the NFL...