When Snow Chief scored a six-length victory in the Santa Anita Derby on Sunday, plenty of people who saw that nationally televised race thought that the Kentucky Derby picture had finally been brought into focus.

On a weekend when almost all of the top 3-year-olds were in action, Snow Chief was the easiest of winners. Moreover, the results of other stakes around the country seemed to confirm the California colt's superiority. Badger Land won the Flamingo Stakes at Hialeah with ease, and Snow Chief has already whipped Badger Land twice this year. Mogambo captured the Gotham Stakes at Aqueduct, and Snow Chief trounced him last month. In view of all this evidence, one newspaper predicted that Snow Chief would be the shortest-priced favorite in the Kentucky Derby since Spectacular Bid.

Yet, contrary to appearances, the results of these major stakes did not establish the existence of a clear order in the 3-year-old ranks, with Snow Chief at the top. In fact, they changed a lot of my ideas about the current generation of Derby contenders. Earlier in the season, I had written that (a) this 3-year-old crop is dismally untalented; (b) the defending champion of the generation, Tasso, is an overrated bum; (c) Mogambo is a bum, too. I hereby retract all these statements. There may be a number of good 3-year-olds capable of whipping Snow Chief in Louisville.

This is a time of year when 3-year-old thoroughbreds can mature and develop very quickly. Sometimes the good ones confound handicappers by improving five or 10 lengths almost overnight. That may have happened in New York.

Tasso and Mogambo had never run particularly fast in their lives and now they were meeting a rival, Zabaleta, who was coming off a brilliantly fast victory in the Bay Shore Stakes.

But Mogambo and Tasso both beat Zabaleta and they did it with impressive efforts. The time of the mile race, 1:34 3/5, was superb. Half an hour earlier, a good field of allowance horses covered the same distance in 1:36 3/5. Mogambo is a much, much better horse now than he was when Snow Chief beat him in the Florida Derby.

Similarly, Badger Land has seemed to improve since Snow Chief beat him. He won the 1 1/8-mile Flamingo in the excellent time of 1:47, confirming that his track-record performance two weeks earlier was no fluke.

While these colts are improving, Snow Chief has kept performing on the same level all season. His Santa Anita Derby victory was not nearly so impressive as it may have looked on television. He was the only speed horse in the race and he was able to take the early lead without pressure, and he saved ground on the rail all the way. Yet his winning time, 1:48 3/5 for 1 1/8 miles, was nothing special.

My speed figures for the top 3-year-olds hardly suggest that Snow Chief is the dominant member of his generation. In fact, he is well down the list:

Mogambo -- 119

Groovy -- 118

Badger Land -- 117

Tasso -- 116

Zabaleta -- 114

Bolshoi Boy -- 110

Snow Chief -- 109

Glider Pilot -- 109

Country Light -- 106

Of these horses, at least a couple -- Groovy and Zabaleta -- wouldn't seem to have the capability to go a mile and one-quarter. Plenty of other colts who once looked like contenders have fallen by the wayside.

Rare Brick, the unbeaten colt based at Oaklawn Park, has become less effective as he tried to go longer distances; his last figure was a 103. And yesterday his trainer announced that Rare Brick will be out a couple of months anyway as he undergoes surgery for a chip in his left front ankle.

Broad Brush and Fobby Forbes, the two Maryland-based contenders, have ratings of 102; they are a cut below the top-class colts.

The critical piece of evidence about the Derby contenders is likely to come on April 19, when Mogambo, Tasso, Zabaleta and others are expected to contest the 1 1/8-mile Wood Memorial at Aqueduct. That race should indicate whether these horses' big performance in the Gotham was a fluke, or whether they have truly developed into the stars of their generation. If so, Snow Chief's chances of winning the Derby are slim. But whatever happens, he is not going to have easy pickings at Churchill Downs.