According to the Cartesian philosophy, matter, in the absence of an immaterial soul, is inert, but living nature poses problems for this view. A principal one was that Cartesians were unable to give a detailed mechanical account of animal life, including the complexities of (apparently) intelligent animal behavior. (Indeed, this remains an unsolved scientific problem to this day.) Another problem was spontaneous generation, the idea, generally accepted at that time, that life can arise spontaneously out of non-living matter (e.g., maggots in decaying meat), and the related problem of the origin of parasites in animals. Eventually (1688) it was shown that spontaneous generation does not, in fact, occur, but parasites remained a problem (often ignored).
A more serious problem was how a complexly structured embryo develops from an apparently simple egg. The difficulty was that, according to Cartesian philosophy, mechanical processes could lead to the increase or elimination of parts already present, but they could not lead the emergence of new, complex structures. Therefore Cartesians developed the astonishing doctrine of preformation: the complete adult is already preformed in the germ cell, and that preformed individual contains preformed germ cells, which in turn contain the preformed children of that individual, and so on. Thus God, at the beginning of the world, had created the preformed bodies of all life until the end of time, as nested, preformed children inside the germ cells of preformed adults. As a consequence, the mechanical philosophy required and implied the existence of God (a point in its favor, from the Church's perspective, compared to the magical philosophy). Although there was empirical evidence against preformationism (e.g. regeneration in crayfish), it was ignored because preformationism seemed to be the only theory consistent with Cartesian philosophy.
One complication of preformationism that could not be ignored was the male and female contributions to reproduction (again). One faction claimed that only sperm contained preformed bodies, so "Adam carried all men in his seed." (Amazingly, demonstrating again how theory can condition observation, early microscopists saw tiny human shapes in sperm!) Other philosophers thought that the mother's eggs contained the preformed bodies, thus granting women a role in reproduction, although merely as an uncreative vessel and nurse, but then they had difficulty explaining the necessity of sperm. According to this view, God had relieved males of the burden of reproduction and the cares of the earth so they could devote their attention to the mechanical exploitation of nature.
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