Glance at some items in the news of late and it seems that many long-held ideas about journalism are unraveling.

Although the economics are still evolving, the Internet has now become a major source of news in America.

For more than two generations, the percentage of Americans reading newspapers has been shrinking. Until 1970 the problem was partially masked by population growth. Overall circulation kept rising. Through the 1980s most of the circulation losses were occurring in afternoon papers. The survivors were stable and financially robust.

Network television news was once the most trusted source of information in America. It also had a monopoly over pictures and television reporting from across the country and around the world.

The convenience of 24-hour cable TV news, offering the latest breaking headlines at any time of the day or night, represents an enormous structural advantage for cable over network television. Cable has become the television news medium of choice. The network most cited as the No. 1 source for news remains CNN, preferred over the broadcast networks and even its cable rivals.

Magazines often are harbingers of change. When large social, economic or technological shifts begin to reshape the culture, magazines frequently are the first media to move, and the structure of the industry is one reason. Unlike newspapers, most magazines are not so tied to a specific geographic area, but are instead centered on interests or niches. Writers are looking for trends. Publishers can more quickly than in other media add and subtract titles aimed at specific audience segments or interests. Advertisers, in turn, can take their dollars to hot titles of the moment aimed at particular demographics.

Radio might be called journalism’s forgotten but stable middle child.