Heredity and environment debaters may have a field day with this one:

Edwin Grosvenor, 27, says he got the idea of starting a new, slick art magazine two years ago. Visiting a Degas exhibit at the Met in New York, he saw potential readers packed six deep trying to gaze at the canvases.

Grosvenor mentioned this to his father, Melville Bell Grosvenor, and faster than you can say National Geographic the two had raised the $750,000 needed to start Portfolio from 14 backers.

Early next month, the bimonthly will be mailed to the 30,000 individuals who have subscribed to it in advance. What they'll receive looks surprisingly like the Geographic - the magazine founded by young Grosvenor's great-great grandfather in 1889, and edited by his father for 11 years - right down to the index printed on the spine.

"People keep the Geographic," says young Grosvenor. "Magazines that people keep have a high renewal rate. So you do everything you can to have people keep it. The spine implies that Portfolio is something that should go on the shelf."

Ed Grosvenor does not have the air of a Washington scion - the perfectly tailored, perfectly groomed, perfectly mannered product that might be expected to issue from the National Geographic Society: slick, exact, years-in-the-making, proofed seven times, noexpenses-spared.

He is more the professional student cum outdoorsman that he was until two years ago: BA in art history from Yale; MA in journalism from Columbia; MBA Columbia; all the while working as a photographer on 12 Geographic stories and at the Miami News. His gray pin-stripe suit is pure Yale; but his has the journalist's demeanor of appearing to have been slept in.

"I didn't get much sleep last night," he says. "In fact, I've been running around so much with the magazine that I haven't gotten much sleep at all." He's downing cups of coffee and thumbing through the 120-odd galleys that have been pasted into a dummy of the first issue; stories on the art of Bellini, Van Gogh, Stella, Fox Talbot.

"Thirty-four and a half pages of ads in our first issue. Not bad. And we'll be the biggest fine arts magazine as soon as we publish - bigger than ART-. news (66,000 circulation) and Art in America (44,000)."

Grosvenor says Portfolio's circulation has been built strictly from mailings to 65 subscriber lists of high profile magazines like New York and Harper's.

"I'd love to use the Geographic mailing list," he says, "but I could never afford to mail out 10 million pieces.

"There's a part of me that wants to stay away from the Geographic, too. You can imagine what it's like being the son of the editor. My father's lent a lot of moral support on this, and he's read a few of the stories, but he really hasn't gotten involved too much.

"The one way this magazine will be like the Geographic is in the timeless quality of the stories. The other art magazines often write about specific exhibits and then they get thrown out when they're over. They also assume that everybody's going to read the stories. We've got a lot of information in our captions. There's even a map here" - in a story of Mexican folk art. "You'd never see that in another art magazine."

Grosvenor says he expects that the magazine will eventually hit a saturation-point circulation of 300,000 - a good way beyond the break-even point of 100,000 he expects to reach before the year is out.

But you get the hunch that he might even go further.

"My grandfather got very frustrated with the Geographic," he says, "because they wouldn't let him send off any letters to solicit new subscribers. They were at about 3,000. One day in 1905 the secretary of the society came in and said he was going on a trip. As soon as he left, my grandfather hired three employes and started sending out letters. When the secretary came back they had 10,000 subscribers."