 The implications of Aristotle’s complex thought for subsequent discussions of species, generated by the recovery of his writings in the Latin West in the twelfth and thirteenth centuries, were varied. On one hand, Aristotle’s apparent metaphysical requirement that the soul-as-form (eidos) be permanent and enduring through the process of the generation of “like by like” seemed for much of the tradition to amount to a denial of the possibility that natural species could change over time in their essential properties, even though local adaptation in “accidental” properties was fully possible. Since individual beings were considered to be dynamic composites of a material substrate and an immaterial and eternal form, the accidental differentiation of the substantial form in individuals did not affect the metaphysical endurance of the species. It also made species extinction metaphysically impossible. In living beings, the soul-as-form is serially passed on through time in the act of generation to create an eternal continuity of the form. This supplied a metaphysical foundation for the notion of species permanence without reliance on an external creative agency.