Advertising has been with us ever since the first chicken clucked when she laid the first egg. But as a business, separated from the product-makers, it is barely 50 years old.

Why did it break away? Why didn't Henry Ford and W. K. Kellogg and their heirs just keep on writing their own ads? Ask Sidney R. Bernstein, the grand old man (at 73) of Advertising Age whose column "con-SID-erations" appears on the editorial page of that half-century-old industry bible.

It began when you got a national audience," he said on a visit here yesterday. "Printing and distribution techniques made it possible to have magazines like Ladies' Home Journal and Saturday Evening Post, which went out to people of similar interests all over the country. But it was expensive to run ads in them. So the companies hired specialists."

There was another reason for the rise of advertising, he said, also rooted in our changing social landscape.

"Always, in the past, you had the man at the counter, you had personal service there in the groceries and drugstores. You'd ask him what was the best coffee or what was good for a headache, and he'd tell you. But with the supermarket and the other big self-service stores, that was all gone." w

It became necessary now to pre-sell the customer "to such an extent," Bernstein noted in a recent article, "that when she enters the place in which actual purchases are made . . . the necessity for personal salesmanship has largely been replaced, and what was once a buying-selling situation now becomes a much simpler order-filling operation."

The recent return to generic products will not greatly affect the market, he added, nor will private house brands.

To the bewilderment of some, in a time when millions of Americans voluntarily wear brand-name belts, hats and shirts and even pay extra for designer luggage, scarves and dresses marked with someone else's name, there is still a concern that advertising is at least subverting American taste and maybe seducing the consumer into the poorhouse.

Bernstein rejects all of this. The brand-name fad is just that, he said, and is not a sinister indication that we are beginning to turn into mere accessories to our own products.

As for the fear of ads' hypnotic power to drive citizens to economic and moral ruin, "advertising is not that good, not that powerful, not that influential."

For one thing, he said, the ads follow taste, not the other way around, very simply because they are trying to sell products.

"You can't sell what people don't want. You can't sell a lemon-yellow whatzit if the consumers really don't like lemon-yellow. It'll never sell."

Furthermore, advertising is a function of a high-level society in which there is a great pool of what he called free income, money not needed for the basics of living.

"As the economy heads into a recession, the jokes and fun and fantasy ads tend to fade out and you see more direct hard-selling, more emphasis on performance."

He showed some 10-year-old Cadillac ads that were pushing social status and some 1979 updates which talked about how efficient the cars were to run.

There have been many changes in the last decade, he told the local Ad Club. Most of all, women are not invairably viewed as housewives: Female customers in the United Air Lines commercials no longer say "Take Me Along," but "I'm the Boss!"

To be sure, the soap and cleaner and house product commercials and many others remain unreconstructed.

"We shouldn't blame ads" for the overwhelming materialism of America, said Sid Bernstein. "By themselves they're meaningless. They are merely one of the tools of marketing. If a better way to sell comes along, it'll be used. . . ."