 This theory of the passing on of soul-as-substantial form in generation also formed the basis of one meaning of “species” (eidos) in Aristotle’s biological works—as the individualized enmattered form-as-soul that is perpetuated in generation eternally. Although connected by a complex theory of mental abstraction to the universal in thought and language, a “species” in this biological sense is not a universal, but an eternal serial sequence of one individual generating another individual (De Anima, II: 415b, 1–10). The importance of this meaning of “species” as distinct from the meaning of “species-as-universal” bears on several issues explored in Sections  3.3  and  3.4  below. One interpretive issue in the exegesis of Aristotle’s conception of species concerns the degree to which he was committed to asserting anything beyond the eternity of the three main groups (gene)—plants, animals and humans—rather than the eternity of each individual kind (eidos) (De generatione animalium, II: 731b, 32–732a5).