Departing from a recent work by Helmut Müller-Sievers the author charts the intricacies of the debate between preformationism and epigeneticism and its theoretico-epistemological repercussions during the eighteenth and nineteenth centuries. Although the most common interpretation equals preformationism to mechanism and fixism, on one side, and evolutionism to epigeneticism and organicism, on the other, the actual picture, once key authors are analyzed, is far more complex. All preformationist theories were, in principle, mechanistic, but not all mechanistic theories were preformationist: they could also be epigenetist, which means that not all epigenetist theories were necessarily organicist. Although all organicist theories were, in principle, evolutionary, not all mechanistic theories were fixist. And finally, all preformationist theories were, in principle, fixist, but not all fixist theories were preformationist. The redefinition of the notion of embryonic preformation in the first decades of the nineteenth-century resulted, in turn, in a new concept of the “organism,” crystallizing a view of nature that combined fixism (at a phylogenetical level) and evolutionism (at the embryological level).