In the eighteenth century, two rival theories of organic generation existed. The 'preformationists' believed that all embryos had been formed by God at the Creation and encased within one another to await their future appointed time of development, while the 'epigenesists' argued that each embryo is newly produced through gradual development from unorganized material. The most important clash between the two schools, the debate between Albrecht von Haller (1708–77) and Caspar Friedrich Wolff (1734–94), crystallized many of the key issues of eighteenth-century biology - the role of mechanism in biological explanation, the relationship of God to His Creation, the question of spontaneous generation, the problems of regeneration, hybrids, and monstrous births. In this book, Professor Roe takes the debate beyond its observational basis and shows that at issue were not only specific embryological problems but also fundamental philosophical questions about the natural world and the way science should explain it.