WHEN FUTURE anthropologists sift through the rubble of late-20th-century civilization and hit the paperback detritus of 1982, they will doubtless conclude that Americans worshiped a lumpy quadruped with a face like a catcher's mitt.

They won't be far from wrong. Garfield, Jim Davis' cartoon kitty, has obsessed the national psyche -- and lightened the national wallet -- beyond the dreams of avarice.

There are now seven Garfield items in print, and tomorrow every one of them will appear on The New York Times trade paperback best seller list. And no wonder: Ballantine has put the cat out with a vengeance. Between the 1980 appearance of the first specimen ("Garfield at Large," 1.26 million copies in print) and this month's latest ("Garfield Treasury," 400,000 copies), about 6.4 million copies have been printed and more than 4 million sold. This despite the fact that all but one ("Here Comes Garfield" consists of story boards from a TV special) have already appeared in newspapers -- 1,200 of them at present. It had "phenomenal syndication growth," says a spokesman for United Media, "the fastest leap from zero to 1,000 papers that any comic strip has ever had." The recycled black-and-white drawings are priced at $4.95 for about 100 pages; "Treasury," which collects the color Sunday strips, is $7.95.

The feline fetish has further produced 1.25 million Garfield calendars and a stupefying mercenary spawn of spinoffs: stuffed dolls, note paper, pencil boxes, coffee mugs . . . virtually every gift shop chattel with a square inch of flat space has been emblazoned with the likeness of the pudgy icon.

How to account for this phenomenon? Davis is no help. The avalanche of pop exaltation has driven him into protective reclusion at his Muncie, Ind., home. "He left strict instructions," says a Ballantine official, "that he doesn't want to be disturbed for a while." And even the pundits are stumped. "I don't know what to make of it," says culture-historian Leslie Fiedler. "It was always the dog at the center of the American folk imagination."

There are market explanations, of course. The strip began in 1978 -- the same propitious year, according to studies by the Association of American Publishers, that trade paperbacks were shifting definitively away from "quality" subjects and toward mass appeal. At about the same time, research was showing that schoolchildren, confronted with paperback and hardcover editions of the same book, assume that the hardcover is more difficult and choose the friendlier paperback. That affinity, coupled with kitty charisma, would make the Garfield items a monster hit with tykes. But "they're selling to an adult market," says Phyllis Ball of the AAP's trade division, despite the fact that unit sales of all adult paperbound books have been dropping recently -- down 9.3 percent from 1980 to 1981. Why adults? And why now?

"The man responsible is Eisenhower," says TV and pop-culture critic Michael Arlen. "Though he had many other fine qualities, Eisenhower was basically elected because of his grin. He was our first cute president." Thereafter, "serious cuteness, once safely confined to the Saturday Evening Post, cocker spaniels and freckle-faced kids, became a national mania." And it remains manifest in everything from "Cats" on Broadway to tots on TV to a president whose notion of statesmanlike oratory is "dipsy doodle."

No way, says G. Roysce Smith, executive director of the American Booksellers Association. "Back in 1949, the number one best seller in trade paper was a book called 'White Collar Zoo,' with photos of animals with word-balloons coming out of their mouths and cute sayings as if they worked in offices." In the '50s, it was "The Baby" -- more pictures, word-ballons and "sassy sayings." Still, Smith finds the prospect of seven Garfields "alarming -- it doesn't leave room for anything else." And although "cartoon books have been on the best seller list for a long time," Smith cannot recall a cat boom before the mid-'70s when B. Kliban laid siege to every book and card shop.

Moreover, Garfield is not exactly cute: He is an arrogant, violent and monomaniacal little mammal, an id-figure whose self-indulgence operates with impunity because -- like his video ancestor, the once and future Morris -- he is presumed to be cute. He is thus the ideal psychic symbol of those adults trying to reconcile the mellow vanities of the Me Decade with the belligerent anxiety of the '80s -- just as his prosocial rodent forebear, M. Mouse, served as the self-image of the '50s.

Nothing new, says Smith. You think diet books are a recent cultural infatuation? Well, "in 1924 and '25, the number one nonfiction best seller was 'Diet and Health' by Lulu Hunt Peters, sort of like the Adele Davis books. It stayed on the list for five years."