 The theory of pre-existence was seen to solve many problems. First, it explained the intimate interrelation of structure and function that seemed to require the existence of parts of the organism in an integrated system. The heart presumably could not beat without ennervation, and the nerves could not exist without the heart. Consequently, as the argument went, the entire organism must pre-exist. The existence of such integrated systems seemed otherwise difficult to explain by the sequential development of parts, as implied in Aristotelian and other “epigenetic” theories of development. Second, this account was easily harmonized with theological developments in the seventeenth century, particularly on the Continent, where the strong influence of Augustinian theology on Calvinism (Protestant) and Jansenism (Catholic) was most in evidence. As an example, it provided for the transmission of Original Sin. The solution that emerged was a “theistic” mechanism that emphasized God’s omnipotence and the corresponding passivity of nature (Deason 1986; Roger 1963 [1997: chp. 6]). A third advantage of the theory of preexistence, at least in the versions that embraced the “ preformed germs” interpretation, was that it allowed for the appearance of life in secular time, which seemed to be suggested by the existence of fossil forms. At the same time no versions of preexistence theory implied any change of species or development of one species from another in time. Finally, some version of a theory of the preformation of the embryo could be reconciled with the best microscopic observations of the late seventeenth and early eighteenth centuries, as these were reported by such experts on this instrument as Anton van Leeuwenhoek (1632–1723), Jan Swammerdam, Marcello Malpighi (1628–1694), and Henry Baker (1690–1774).