Book excerpt:  By examining important aspects of science fiction in the twentieth century, this book explains how the genre evolved to its current state. Close critical attention is given to topics including the art that has accompanied science fiction, the subgenres of space opera and hard science fiction, the rise of SF anthologies, and the burgeoning impact of the marketplace on authors. Included are in-depth studies of key texts that contributed to science fiction's growth, including Philip Francis Nowlan's first Buck Rogers story, the first published stories of A. E. van Vogt, and the early juveniles of Isaac Asimov, Arthur C. Clarke and Robert Heinlein.