The boys of summer - in the fall

Baseball's drive to finish first now has accelerated. Four teams unaccustomed to national glory have begun best-of-five playoffs; not far behind is the seasonal climax, the best-of-seven World Series. Americans, often indifferent during summer, are poised to show why the sport is called the national pastime.

This year it's a bit bewildering for fans accustomed to rooting for the underdog: Each of the four has a claim on that role. Detroit long has been an also-ran, Kansas City has overcome extraordinary adversity, and San Diego never before made it to post-season play.

But the champion underdog has to be Chicago's Cubs: 39 years a nonwinner, they zoomed from last place in '83 to first this summer. Their presence in this year's round-of-four means postseason baseball is once again being played the way Abner Doubleday doubtless en-visioned it: in the daylight. The Cubs' Wrigley Field is the only major league stadium without lights. There's something to be said for anachronisms.

Finishing first takes talented ballplayers. And, despite the judgment of Lippy Leo Durocher, it's something nice guys can do. One of the champions at finishing first was Walter Alston, who passed on this week. He won seven league championships and four World Series in 23 years as manager of the Los Angeles Dodgers and their storied predecessors, the Brooklyn Dodgers - familiarly known to the Flatbush Faithful as Dem Bums. He was a person of quiet dignity respected as, in the words of Dodger owner Peter O'Malley, ''a first-class gentleman.''

This year's drive to world series supremacy began under threat of an umpires' strike. Lately the umps have joined players in insisting on a bigger cut of the multimillion-dollar gate receipts; in 1979 they briefly took a walk. Once again this week league presidents had to scout around for amateur umps just in case.

At any rate, the last round of ''play ball!'' punctuates the change of seasons on the athletic calendar. Football has begun, hockey is about to start, and basketball teams are training now.

For the sports fan, it is a menu rich in choice.