Of God and His Creatures

It will be said, -- An animal moves towards maturity and full strength: but therein it does not rest: it moves just as rapidly out of its perfection as it moved into it. Not quite so: there is what is called hêlikia kathestôsa, 'settled age,' like settled summer weather; and the more perfect the animal, the longer this settled age endures.

Of God and His Creatures: 3.16