A link in the evolution of black popular music from the '60s sounds of Atlantic, Motown and Stax to those of Philadelphia, Parliament/Funkadelic and disco in the '70s exists on the albums made by Sly & The Family Stone at the turn of the decade.

If Sly Stone, along with Stevie Wonder, had not fused R&B funk flowerpower rock, it is questionable whether the homogenized strains of Earth, Wind & Fire, the Bee Gees or Donna Summer would exist today.

But although Sly had his share of hits (he earned no less than nine gold records, with the breakthrough "Stand!" album remaining on the charts for more than two years), only recently has his pivotal role in the development of American popular music been recognized. As if in response to this recognition, Sly has two new releases which, while they may not put him back in the creative forefront, will certainly solidify his standing as one of our most intriguing and important black recording artists.

After 1971's bleak but brilliant "There's a Riot Goin' On," Sly's albums turned lightly philosophic, moving away from sweeping social statement toward more personal, subjective material. "Back on the Right Track" (Warner Bros, BSK 3303) continues in that vein, with such self-explanatory titles as "It Takes All Kinds," "Who's To Say?" and "Remember Who You Are." The latter, released as a single, is the LP's best cut, alternately laid back and rousing, with the backing chorus commanding "Remember!" in high register as Sly growls knowingly, "who you are . . . "

Also first-rate are "Who's To Say?" with its clever vocal arrangement (always a Sly forte), and the title track, powered by the interweaving of riffing horns, rollicking guitar and stuttering organ. But even these choice cuts are somewhat thin, begging for a fuller, more dynamic production. There's no guessing the problems producer Mark Davis might have encountered, but he doesn't seem to take full advantage of all the studio weapons at his disposal in trying to make "Back on the Right Track" sound contemporary.

Which raises the question: Has Sly been listening to what's been going on around him the past few years? Much of his early power came from his ability to draw from disparate rock, jazz and R&B elements, but there is little on this album to indicate that he has remained in touch. One could listen to "Back on the Right Track" and his previous LP, 1976's mediocre "Heard Ya Missed Me, Well I'm Back," and never guess that anything akin to disco or new wave had become popular in the interim. Of course, this is partly to Sly's credit; even if the vocal wha-wha on "The Same Thing (Makes You Laugh, Makes You Cry)" or the largely instrumental filler "Sheer Energy" sound dated next to today's pyrotechnic production values, at least Sly hasn't tacked on the obligatory disco beat in order to ingratiate himself with a new, younger audience. "Back on the Right Track" may not be the mindblowing comeback album we might have hoped for, but neither is it the sellout we might have feared. It is simply a credible comback.

But if Sly hasn't capitulated to the disco beat, Epic Records has. That, in a nutshell, is what the company has done with several of Sly's greatest hits on "Ten Years Too Soon" [Epic JE 35974]. The theory was that remixed and strategically re-recorded versions of classics such as "Dance to the Music." "Stand!" and "Everyday People" would make inroads with the hard-core disco market.

Esthetically speaking, it is a travesty. Not that re-mixer and editor John Luongo has done a hatchet job -- on the contrary. In respect of trying to make the old hits viable with the sophisticated sound systems in the discotheques, Luongo was reverential in his handling of the material. But any attempt to take original master recordings and overhaul them to conform to prevailing musical tastes, without the involvement of the artist, must be seen as abhorrent. There are rumors that Sly will sue Epic, but they are as yet unsubstantiated.

Those who remember or still listen to the original versions will find "Ten years Too Soon" difficult to embrace. Of the seven tracks, "Sing a Simple Song" and "I Get High On You" (only hand claps were added to the latter) make the smoothest transition to disco but it remains a matter of Luongo's doing a relatively unobtrusive job of putting Humpty Dumpty back together again. On the whole, in taking these songs apart and artifically injecting incessant kick-drum and percussion (as well as some new guitar parts), Luongo loses their exuberant charm and much of their impact. But the discos demand only that the beat be consistent and it could be that this material is strong enough to survive the transfusion and make another run at the charts, despite the loss of lifeblood.

With the exception of disco, popular music seems, distressingly, to have segregated itself in recent years. This is evident at concerts -- where black artists often draw predominantly black audiences and white artists attrack mostly whites -- and in the pattern of record sales. This state of affairs might leave Sly Stone without the widespread integrated following he once enjoyed -- and helped to create. "Ten Years Too Soon" and "Back on the Right Track" are both true to their titles, but it remains to be seen whether either will find its intended audience.