The Unfit, by Elof Carlson, explores the sources of a movement--negative eugenics--that was used to justify the Holocaust, which claimed millions of innocent lives in World War II. The title reflects the nearly three centuries of belief that some people are socially unfit by virtue of a defective biology, and echoes an earlier theory of degeneracy, dating to biblical antiquity, in which some people were deemed unfit because of some transgression against religious law. The author presents the first biological theory of degeneracy--onanism--and then follows the development of degeneracy theory throughout the nineteenth century and its application to a variety of social classes. The key intellectual theories and their proponents form the framework of this exploration, which includes the concepts of evolution and heredity and how they were applied to social problems. These ideas are followed into the twentieth century with the development of theories of positive and negative eugenics, the establishment of compulsory sterilization laws, racism and anti-Semitism, and the Holocaust. This story of misapplied science and technology is one that still haunts humanity in the twenty-first century. The ghost of eugenics recurs in many guises during debates and controversies about intelligence testing, genetic screening, prenatal diagnosis, gene therapy, new reproductive strategies, and uses of our genomic information. Carlson ends his discussion of the history of humanity in this arena with an exploration of the future of genetics that is based on new technologies and application of the Human Genome Project findings, as well as a discussion of the death of the old eugenics and of the problems that will not go away, including our ambivalence about our own biology.