The philosophy of Raymond Ruyer was an important if subterranean influence on twentieth-century French thought, and explicitly engaged with by figures such as Maurice Merleau-Ponty, Georges Canguilhem, Gilbert Simondon, and Gilles Deleuze. The Genesis of Living Forms is Ruyer’s most focussed and forceful analysis of a central but apparently paradoxical biological phenomenon that also presents serious problems for philosophy: embryogenesis. When a cat develops from the early stages of fertilization to an adult, what is it that makes it the same cat? How is it that a living being can at once be the same and constantly changing? Ruyer’s answer to these questions unfolds through a detailed set of encounters with major scientific fields, from particle physics to social psychology, arguing that the paradox can only be dissolved by seeing the role that form plays in the ongoing development of living beings. In Ruyer’s view, embryogenesis is a central problem not just in the life sciences; every thing must possess a relation to a form that is characteristic of it, from carbon atoms to embryos, and to embryologists themselves.