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...