WORLD: Stuart Elliott in America.

In recent years, US broadcast network television has started to resemble its British counterparts more closely. Pop Idol inspired American Idol. Who Wants to be a Millionaire? was cloned across the Atlantic and NBC purchased the rights to make Yankee-fied versions of Coupling and The Office. Now the trend is accelerating faster than expected, expanding beyond content to embrace form.

For decades, the US broadcast networks were the unquestioned masters of their media domain. The templates they adopted, based on their radio roots, were copied assiduously by those who followed. At the heart of it all: an annual primetime season, running from September through May, focused on regularly scheduled series. The dramas, sitcoms and other shows appeared only once a week, on certain evenings and their dozens of episodes were not repeated until months later.

That model served the broadcast networks well, drawing tens of millions of viewers and generating billions of dollars in advertising revenues.

Indeed, as recently as last spring, the major broadcasters such as ABC, CBS, Fox and NBC sold a record dollars 9.3 billion-worth of commercial time ahead of the coming fall season.

But cracks in the foundation of the broadcast system, once of hairline width, are becoming fissures and in some cases assuming Grand Canyon-like proportions. Among the problems: falling Nielsen ratings; the loss of desirable demographic groups such as younger men to the internet and video games; an upsurge in viewership for cable networks and satellite TV and, perhaps most alarming, the growing popularity of reality-TV series, which work well in short bursts but less so in ...