The generation of animals was a difficult phenomenon to explain in the seventeenth century, having long been a problem in natural philosophy, theology, and medicine. In this paper, I explore how generation, understood as epigenesis, was directly related to an idea of rational nature. I examine epigenesis—the idea that the embryo was constructed part-by-part, over time—in the work of two seemingly dissimilar English philosophers: William Harvey, an eclectic Aristotelian, and Margaret Cavendish, a radical materialist. I chart the ways that they (...) understood and explained epigenesis, given their differences in philosophy and ontology. I argue for the importance of ideas of harmony and order in structuring their accounts of generation as a rational process. I link their experiences during the English Civil war to how they see nature as a possible source for the rationality and concord sorely missing in human affairs. (shrink)