Dance critic Siegel ""began this book because of a desperate and continuing sense that not enough was being done to impede the extinction of yesterday's dance."" And the valuable pages here are the ones that describe, subjectively but with more clarity and detail than eloquence, a number of rarely seen early modern-dance works going back as far as Doris Humphrey's 1928 Air for the G String. Notwithstanding the increasing use of videotape for preserving dance, students and performers will appreciate Siegers synopses of a half-dozen Humphrey pieces, a dozen by Martha Graham, and more recent work by Merce Cunningham, Eliot Feld, and Paul Taylor. Somewhere along the way, however, Siegers book became more ambitious: the dances are grouped by style, theme, or choreographer (""Neoclassicism I,"" ""Americana,"" ""Balanchine's America""), and Siegers connective commentary attempts to pass judgment on various talents, to trace influences and make comparisons, to fashion a freeform history of the last 50 years of American dance. In this effort, she is far less successful Even a brilliant critic would find it difficult to develop themes through such an oddly organized format, and Siegel is less than brilliant; her serviceable prose rarely delivers fresh insights and frequently lapses into thundering banalities (""Angst . . . is a very real component of the American psyche in the twentieth century""), fatuous pronouncements (""What is most important about Tharp, although it's often forgotten, is that she's fundamentally a serious artist""), unhelpful generalizations (""Graham's dances are self-directed, Tudor's are other-directed""), and talk of ""cosmic levels."" Her views of Agnes de Mille (""Hollywood"") and Jerome Robbins (""above all else, a theater man"") are severely limited, and Balanchine has been distilled far more vividly elsewhere. Useful in spots, then, but no substitute for concentrated studies of individual choreographers or a full-scale critical history.