Mr. Clemens began to write his autobiography many years ago, and he continues to add to it day by day. It was his original intention to permit no publication of his memoirs until after his death; but, after leaving ''Pier No. 70,'' he concluded that a considerable portion might now suitably be given to the public. It is that portion, garnered from the quarter-million of words already written, which will appear in this Review during the coming year. No part of the autobiography will be published in book form during the lifetime of the author.