AMERICAN SAMOA, according to Federal Judge William B. Bryant, has come of age. Fa'a Samoa, the words Margaret Mead taught us to use when we refer to the Samoan way of life, has yielded to the 20th century and Americanization. The matais (leaders) no longer dominate the lives of the members of their families and even aiga (family ties) have lost their hold. If Judge Bryant is right, a social system that had endured for ages is gone.

You may well wonder what a federal judge has to do with deciding when a particular way of life has come to an end. That's something usually, and more appropriately, left to the sociologists and anthropologists. But the United States has legal jurisidiction over the Samoan Islands, and the Constitution follows the flag. Judge Bryant was required to decide whether an American citizen could insist upon a jury trial in a Samoan court. To decide that, Judge Bryant had to determine whether the circumstances of modern life in Samoa make trials by jury practical.

It was those circumstances of life - as they were in the 1920s - that Margaret Mead described so brilliantly in her classic book "Coming of Age in Samoa." And it was from her work about fa'a Samoa than most of them ever wanted to know. But Dr. Mead's own testimony that the stratification of Samoan culture would make the selection of impartial juries impossible was not persuasive to Judge Bryant. The Americanization of Samoa, he ruled, has been too pervasive. Television, compulsory education and even football have come to it.Its government structure is much like ours, and its legal system, with the exception of juries, is almost identical to ours. Now that exception will disappear.

We don't know whether to be sad or glad at this broad change in Samoan life that Judge Bryant perceives. Something that was fascinating - and seemed so permanent when some of us read about it in college - is gone. Something that Americans, at least, think is good has replaced it. Where the balance lies we do not know. It will take another great book - "The Coming of America to Samoa" - to help us make that judgment.