St. Thomas points out that a thing cannot simply be said to be good in the same way that it is called being. Being is said in the first place of the substantial being of things, and only secondarily, in a more limited sense, of the accidental being, which is added to a substance. But because goodness of its nature implies perfection it is attributable most of all to that by means of which a thing attains its perfection. Now this occurs more through accidental determinations, such as health, development of the mental faculties and virtues; being the end desired by other things also ranks high in what we call being perfect. Perfection presupposes unity: the more unified something is the more perfect it becomes. This means that the good is not just the static possession of one’s ontological content and perfection, it is also that which is still to be attained. Creatures and in particular man must further develop and perfect themselves. In this way the good also shows the character of a fullness of being still to be reached. …
The principle that good spreads itself (‘bonum est diffusivum sui’) must be understood exclusively of final causality. Likewise Augustine’s words “Because God is good we exist” (‘De doctrina christiana, I, 31) must be interpreted as referring to the final causality of God’s will. St. Thomas rejects the explanation of the above-mentioned principle through efficient or, a fortiori, mechanic causality. On the level of efficient causality the good is not active, for it is in the nature of the good to perfect other things in the manner of the end. Needless to say that Aquinas does not deny that good things also act as efficient causes. But this does not imply that the good as such is an efficient cause. The Platonic principle of the good spreading itself tends to reduce the causality of the good to a necessary natural activity, just as for the Neoplatonic philosophers the emanation of the hypostases is indeed a necessary process which they illustrate by the image of a gushing well or a source radiating light.
Now, here’s a thesis for the disputatio:
“The Darwinian theory of propagation by natural selection is but the ecological restatement of the classical metaphysical principle that bonum diffusivum est sui.”