Evolutionary theory has long been debated, not only on religious grounds, but in terms of its usefulness as a scientific model. A common objection is that the theory does not fit the hypothetical-deductive standard used in the physical sciences and is therefore not "good" science. In this study, Lloyd contends with the scientific and philosophical critics of evolutionary theory. Employing the same type of semantic analysis that has been applied to physical theory, she offers an alternative view of evolutionary theory that shows biological and physical theories to be structurally similar. The author begins by reviewing recent debates concerning the existence or non-existence of evolutionary "laws" and the possibility of axiomizing evolutionary theory. The central discussion of Lloyd's work revolves around several problems that have not been resolved by other methods. These are, first, the problem of which "units" or entities are selected in nature; second, the problem of reductionism in genetics; and third, the problem of confirmation. Developing a view of evolutionary theory based on mathematical models, she uses this framework to describe a wide variety of evolutionary subtheories. She tests her analysis by comparing it with other approaches and applying it to problematic cases. Finally, she presents a view of confirmation appropriate to a model-oriented conception of theories. This original study explores some significant connections between science and philosophy and adds to our understanding of the nature of scientific knowledge.