IONCE had a theory that country music was invented by AM-rock songwriters to make their product sound intelligent by comparison. After all, not even "Jeremiah was a bullfrog..." would find Tammy Wynette a hard act to follow. And if lyrics like "I beg your pardon,/ I never promised you a rosegarden..." wouldn't send you begging for a copy of James Brown's "Hot Pants," it's doubtful that anything would. Good country music has largely been a triumph of performance over material, and there is no question that any first-rate country guitarist, fiddler, or banjo player could put most rock musicians to shame.

Doc Watson is probably the greatest living flat-picking guitar player. His performances are invariably distinguished not just be occasional bursts of dazzling speed or by audience-winning tricks, but by an unerring sense of timing and mood which might enable him to make a country rendition of the Boston Yellow Pages sound interesting. His smooth, if not polished singing style offers relief from country singers who sound something like the cows they are singing about, and his interpretations of contemporary folk music are as successful and perhaps even more pleasing than his classic renditions of traditional country romances, evangelical songs, and blues.

Since 1964, Doc has toured with his banjo-and-guitar-playing son Merle, whose growth as a musician and contribution to Doc's music have been considerable. With Doc going on 50 and Merle not quite half that age, the younger Watson is still overshadowed by his father on guitar, but impressive on banjo and a promising arranger of country tunes.

Then and Now is the newest Doc and Merle Watson collection, and the first to give Merle equal billing with his father. It's only fitting, since after ten years the two play so well together, exchanging melody and harmony, lead and rhythm, foreground and background parts so cleanly and delicately that they produce what sounds like a single guitar playing impossibly intricate music. The sound is light, the rhythm flawless, and the product a source of inspiration to country-rock musicians from Jim Messina to Jerry Garcia.

THE FIRST SIDE of Then and Now is apparently "Now." It includes a pretty version of Townes Van Zandt's "If I Needed You," a fairly conventional love song that Doc and Merle's performance and a restrained string arrangement by Chuck Cochran turn from maudlin into lively. The Watsons are equally successful with Tom Paxton's "Bottle of Wine." Merle's instrumental arrangement "Bonaparte's Retreat" shows off not only the precision of his and his father's guitar playing, but also the unusually expressive fiddling of Vassar Clements. Clements demonstrates that a fiddle can make you feel things besides the urge to stamp your foot, and, in other songs, his surprising changes in rhythm and tone are even more pleasing than the theatrics of most lightning-fast fiddlers.

The second side comprises a set of older songs, the most interesting of which is a rendition of "Frankie and Johnny," a chance for Doc, Merle, Vassar Clements, and dobro player Norman Blake to show off on brief solos. Again, in this set, the punctuation of the music with brief bursts from Clement's fiddle or from Doc's harmonica is often enough to make simple music interesting. The rest of the songs, though bright, energetic, and pleasant to listen to are less invigorating; they are so standard that they elicit no subtle vocal interpretation.

Then and Nowsucceeds in conveying the strengths of country music--the feel of good times, the directness of sentiment, the considerable energy and warmth of performances too lively to sound static, but too intricate to be completely improvised. The two main disappointments in the album are its length and range. Both sides combined add up to less than a half hour of music, and none of the songs are triumphs as unusual or moving as Doc Watson's earlier rendition of Gershwin's "Summertime" or as instrumentally demanding as the classic "Black Mountain Rag."

It's impossible to conceive of a Doc Watson album that wouldn't be bright enough to ward off an average case of the doldrums, but, as pleasing as this album is, Doc and Merle Watson are capable of much more. That, in itself, is a testimony to their skill, that even a better-than-good album is ordinary for them, and just lively, catchy performances seem too little of a challenge.