The perfectibility of the human species remains one of the most controversial subjects in science, and Maranto, an award- winning science writer, explains why. She begins with a look at a meeting of the cutting-edge specialists in ``assisted reproduction.'' Physicians whose concern is to help infertile couples achieve parenthood, whether through artificial insemination, hormone treatment, or in vitro fertilization (IVF), are the new medical elite. Their specialty grows out of the oldest of human concerns: the regulation of the population. The author traces back to ancient societies the practice of infanticide and the enslavement of abandoned children. But the arrival of modern biology brought the eugenics movement, proposing that regulation of human breeding was as natural as the selective breeding of livestock. While respected scientists endorsed the concept, Maranto argues that its logical conclusion was the Nazi extermination of ``undesirables.'' Meanwhile, research on human fertilization and conception was proceeding. The first recorded application of artificial insemination to humans was in 1884, when Dr. William Pancoast of Philadelphia performed the procedure--with the permission of neither the woman or her husband. The growth of the practice was controversial with legal and religious authorities. As with most of the other practices Maranto discusses, doctors were generally content to follow the available technology to its natural conclusion. Further controversy followed the development in the 1970s of IVF, and the legal tangles accompanying surrogate motherhood are still unresolved. The new cutting edge is genetic therapy, which promises to eliminate such diseases as Tay-Sachs and sickle cell anemia, and could allow parents the dubious ability to choose traits such as skin color and gender. While Maranto scrupulously presents the views of all sides, it is clear that her own position is that science has gone too far. A comprehensive, passionate, and thought-provoking look through the door into a brave new world in which we may find ourselves before we realize it.