In comparison with the reductive theories of Aristotle's predecessors, Aristotle's ontology is very full. He takes it as an undeniable fact that medium-sized objects of experience really do come to be and perish. Their appearing to do so is not reducible, as the materialists would have it, to changes in position of more basic material particles. Medium-sized objects are "substances."

Living organisms are paradigm instances of Aristotelian substances. Aristotle takes it as a further, undeniable fact that organisms regularly produce other organisms that are the same in kind or species: Human begets human, not dog or fish.

These facts are not explicable by the movements of more basic materials, nor are they explained by the relation that material substances stand in to an immaterial, separately existing Platonic Form. Rather, Aristotle explains the regular reproduction of conspecific organisms of the same species in terms of the transmission of form from one generation to the next. A form at the level of the species, present to the matter as an organizing principle, plays an indispensable causal explanatory role.

Given this indispensable role for forms in explanations, Aristotle's confidence in the superiority of his ontology - one that countenances forms in addition to matter that the forms organize - appears warranted. The inclusion of form in his ontology is justified by the explanatory work that forms do. This justification for forms is threatened, however, by the current consensus on Aristotle's Generation of Animals. Scholars think that the form that is actually used in Aristotle's scientific explanation of animal reproduction is not the same as the form in his Metaphysics. The dominant reading of Generation of Animals is that it employs a "sub-specific" form, one that varies from one individual to the next. This reading is not only in tension with Aristotle's Metaphysics, but I argue, internally inconsistent. I argue for an interpretation of the theory of reproduction in Generation of Animals that avoids these problems, by assigning to species form a privileged causal role in generation.