 It is in these discussions that we encounter the first early modern systematic speculations connecting naturalistic theories of earth history, embryological development, and changes of organic species in response to external changes of conditions of life. These connections develop directly out of critiques of the preexistence theory of generation by Maupertuis and Buffon, who in different but related ways reformulated the “mechanistic” epigenetic embryology previously advocated by Descartes and Atomist theorists. Similar to these discarded theories (see above,  Section 2.2),  the existence of equivalent male and female “seeds” was postulated, which were then assumed to combine in sexual generation to form the embryo. The new eighteenth-century versions supplemented these seventeenth-century accounts with a novel role for dynamic conceptions of matter and inherent organizing forces. In the case of Buffon, these claims were supported by controversial microscopic observations that presumably refuted the opposing claims made by William Harvey a century earlier.