As early as 410 A.D., then, the greatest of the Western Church Fathers was telling us that the Book of Genesis is not an astrophysics or geology textbook. Augustine himself was a kind of evolutionist, speculating that Gods creation of the cosmos was an instantaneous act whose effects unfolded over a long period. God had planted rational seeds in nature which eventually developed into the diversity of plants and animals we see today. St. Thomas Aquinas cites this view of Augustines more than once in the course of the Summa Theologiae. St. Thomas, author Etienne Gilson writes,
was well aware that the Book of Genesis was not a treatise on cosmography for the use of scholars. It was a statement of the truth intended for the simple people whom Moses was addressing. Thus it is sometimes possible to interpret it in a variety of ways. So it was that when we speak of the six days of creation, we can understand by it either six successive days, as do Ambrose, Basil, Chrysostom and Gregory, and is suggested by the letter of the text . . . Or we can with Augustine take it to refer to the simultaneous creation of all beings with days symbolizing the various orders of beings. This second interpretation is at first sight less literal, but is, rationally speaking, more satisfying. It is the one that St. Thomas adopts, although he does not exclude the other which, as he says, can also be held.
In this century, Cardinal Bea, who helped Pius XII draft Divino Afflante Spiritu, wrote that Genesis does not deal with the true constitution of visible things. It is meant to convey truths outside the scientific order.