Thomas S. Kuhn's "The Structure of Scientific Revolutions" describes the cyclical process by which science develops. This process, far from one of slow, gradual accumulation, is a process of revolution in which one framework for scientific thought is continually displaced by another. Its beginning is marked by the establishment of a paradigm, which allows for normal science to occur. Normal science illuminates anomalies, which may be resolved under the established paradigm, shelved, or deemed significant enough to cause a crisis. If a crisis results, a scientific revolution soon follows, and a new paradigm is established. The process then repeats itself. An example that illustrates Kuhn's model well is the replacement theory of spontaneous generation with the theory of biogenesis, which revolutionized the field of microbiology.