 Concerning the issue of species-permanence and origin, Aquinas treated this in dialogue with Augustine’s theory of serial creation. As he comments in his major synthesis, the Summa Theologiae: “New species, if they appear, pre-exist in certain active powers” (Pars I, Qu. 73, Rep Obj. 3). This claim suggests a reading of Aristotle that assumed the essential fixity of each definable substantial form, and the eternity of each species since creation. But this conclusion must be read in the context of Aquinas’s complex theory of creation and the discussions of the problem of universals in late Scholastic philosophy (see the entry  medieval problem of universals,  and Wilkins 2009: Ch. 3). This context prevents any simple picture of Scholastic thought concerning the issue of species permanence, and it cannot be claimed that either Aristotle or later Scholastics such as Aquinas are responsible for the strong “essentialist” position often attributed to them in the literature (R.A. Richards 2010: chp. 2). As developed below, it can be argued that the species concept was “hardened” only in the early modern period with the rise of the mechanical philosophy and its attendant preformationist embryology. The “strong essentialist” position often attributed to Aristotle and Scholasticism (Hull, 1992), is demonstrably a product of later historical developments (R.A. Richards 2010; Wilkins 2009; Oderberg 2007: chp. 9; Winsor 2006; Lennox 1985, 1987).