First, are there really substantial mutations in natural bodies? Yes, and it can be proved.⁶⁰ Non-living bodies differ substantially from living bodies. But many non-living bodies, by way of nutrition, are transformed into living bodies. Therefore, there are substantial mutations in natural bodies.

The major premise is evident from the many and profound differences between living and non-living bodies, that no one can maintain there are only accidental differences. The minor premise is also certain, for non-living things do actually participate in life after assimilation in nutrition. Therefore, there are substantial mutations in natural bodies.

Second, is it correctly inferred that all natural bodies consist of two substantial principles, one potential and the other actual? Yes, and it can be proved.⁶¹ Two things are required in every substantial mutation: first, a subject which is the body in potency; and formal terminals (termini formales) of mutation through which what is in potency is actuated and determined to this or that species of bodily substance. But in substantial mutation, the subject, the term from which (a quo) and the term to which (ad quem) must be in the genus of substance, as is obvious. Therefore, natural bodies consist of two substantial principles, one potential and the other actual.

Third, are the two substantial principles, one potential and the other actual, really distinct between themselves? Yes, and it can be proved.⁶² It is evident that the potential principle is really distinct from the actual principle, because the material is the same in both terminals of transmutation; but the forms of that material are truly diverse, since one disappears with transmutation and the other form begins to exist. Confirmation of this is had from inverse mutation, which happens at the death of a living body; in death, the living body regresses to a non-living body.

The proof of the thesis that Evolutionism⁶³ is compatible with Hylemorphism can be stated syllogistically.

The first argument is from the Principle of Causality. The effect must be in proportion to its cause. Hylemorphism (cause) explains substantial change (e.g., change to a new species) (proportionate effect). Therefore, Hylemorphism can be the proportionate cause of substantial change. Evolution needs a proportionate cause for the substantial change to new species. Therefore, Evolutionism is compatible with Hylemorphism

The major premise of the above argument is the principle of causality. The minor premise is proved metaphysically because Hylemorphism (formal cause and material cause) explains substantial change (proportionate effect). The minor premise is also proved metaphysically because the Hylemorphism provides both the act and the internal constituent of individuation for being this specific kind of species. The conclusion follows that Hylemorphism is a proportionate cause of new species. Since Evolutionism deals with new species, it is fittingly compatible with Hylemorphism.

The second argument is from the Principle of Sufficient Reason. A sufficient reason is needed for substantial change. But Hylemorphism provides a sufficient reason for substantial change to a new species. But again Evolutionism involves substantial change to new species. Therefore, Evolutionism is compatible with Hylemorphism.

The major premise of the argument above is the principle of sufficient reason itself: Nothing exists without a sufficient reason.⁶⁴ The major premise asserts that substantial change exists,⁶⁵ and that higher grades of life exist.⁶⁶ The first minor premise philosophically explains Hylemorphism has the elements necessary for substantial change to new species; these are act and potency. The second minor premise notes that Evolutionism involves substantial change to new speices. Therefore, Evolutionism is compatible with Hylemorphism.