 The  teleological  orientation of Aristotle’s biology means that he is often characterized as an ontological anti-reductionist (e.g., the elemental potentials of lower levels of organization are for the sake of but do not determine the uniform parts at higher levels of organization). This blends with epistemic issues related to particular domains, such as his rejection of Presocratic materialist (i.e., purely “mechanistic”) explanations for the origin of animal morphology during development. (Empedocles apparently claimed that vertebrae arise during ontogeny solely as a consequence of a unified cylindrical column physically breaking into the observable iterated units of the backbone; see Parts of Animals I.1 [640a].) Yet Aristotle was attentive to issues of material composition and constraints arising from the nature of properties exhibited by animal parts, which often mark commitments to types of ontological and epistemic reduction. His concept of hypothetical necessity (Physics II; see Cooper 1987) permitted the use of one kind of material in many different organismal parts. Galen, among many others subsequent to Aristotle, also explored these issues, whether from the perspective of how material complexes generate higher level properties (Mixtures) or the relations among parts and wholes in the context of development (The Construction of the Embryo).