1,600 years ago, Augustine (354-430) debated with people who did not believe in the historical and allegorical worldwide flood that is documented in the ancient Book of Genesis.

Of the Ark and the Flood; and that we cannot agree with those who accept the historical meaning of the story but reject its allegorical meaning, nor with those who accept the symbolic meaning but not the literal.

The Isaiah scroll (1QIsa^a) of the Dead Sea Scrolls contains almost the whole Book of Isaiah.  Augustine views books like Isaiah as, "books which have been so religiously preserved for thousands of years."

No one, however, should suppose that the story of the Flood is simply without purpose; or that we should seek in it only a true historical account without allegorical significance; or, conversely, that the events recorded in it are entirely unhistorical, and the language merely figurative; or that, whatever else it is, the story is not a prophecy of the Church.  For who save one with a perverse mind would contend that books which have been so religiously preserved for thousands of years, and with such concern on the part of their guardians for a well-ordered transmission, were written to no purpose, or that they are to be consulted for the sake of historical facts only?  To say nothing of anything else: if it was the large number of animals that compelled Noah to make an Ark of such great magnitude, what compelled him to put into it two of each unclean animal, but seven of each clean kind, when both kinds could have been preserved by the same number?  Moreover, although God required them to be preserved in this way in order to renew their species, did He therefore not also have the power to re-create them in the same way as he had first created them?

Some contend that the Flood is not historical

The Deluge, John Martin (1789–1854).

We come next, then, to those who contend that the story of the Flood is not historical, but consists merely of things symbolic and figurative.  First, they maintain that it is not possible for there to be a flood so great that the waters rise to a height of fifteen cubits above the highest mountain tops; for, they say, clouds cannot gather above the summit of Mount Olympus, because that summit is already located so high in the heavens that the denser air in which winds, clouds and rain are formed is absent.  But they do not notice that earth, the densest element of all, can exist there; or will they perhaps deny that the summit of the mountain is made of earth?  Why, then, do they admit that earth is allowed to rise so high into the realm of heaven, and yet contend that water cannot be allowed so to rise?  For those who measure and weigh the elements tell us that water rises higher and weighs less than earth.  What reason do our adversaries offer, then, to explain why earth, a heavier and lower element, should have been invading the more rarefied region of heaven during the revolution of so many years, yet water, a lighter and higher element, was not permitted to do so even for a short time?

Some say the Ark could not have contained so many animals

Noah's Ark (1846), a painting by the American folk painter Edward Hicks.

They say also that the Ark could not have contained so many animals of both sexes, two of each of the unclean and seven of each of the clean.  But it seems to me that our adversaries are here counting only the 300 cubits of length and the fifty of breadth, without noticing that there is the same amount of space on the storey above, and the same amount again on the storey above that, so that, multiplied by three, the dimensions of the Ark come to 900 cubits by 150.  Let us also note the suggestion not inappositely made by Origen: that Moses, the man of God, was, as it is written, 'learned in all the wisdom of the Egyptians' (Book of Acts 7:22), who loved geometry, and that he therefore may have meant geometrical cubits, one of which is said to be equal to six of ours.  Who, then, does not see how many things an Ark of such great capacity might hold?

Some say it would not be possible to assemble an Ark of such magnitude

The Building of Noah's Ark (painting ascribed to a "French master" of 1675)

It is also argued that it would not be possible to assemble an Ark of such magnitude; but this is a falsehood, and a very inept one.  For our adversaries know that immense cities have been constructed, and they fail to notice that a hundred years were spent on the building of the Ark.  Now if it is possible for stones to adhere to one another when cemented merely by lime, to make an encircling wall many miles in length, why is it not possible for timbers to be joined together by pegs, bolts, nails and bituminous glue, to make an Ark extending to such great length and breadth in straight, not curved, lines?  Such an Ark would not require to be launched into the sea by human effort, but would be lifted up by the water when it came, because of the natural difference in weight; and, when afloat, it would be steered by divine providence rather than by human prudence, lest it incur shipwreck anywhere.

Some say that insects would have been overrepresented on the Ark and that aquatic fish and birds could not live on the Ark

The Dove Returns to Noah, c. 1896-1902, by James Jacques Joseph Tissot (French, 1836-1902)

Next comes a question often asked by the excessively pedantic, concerning the tiniest of creatures: not only such things as mice and newts, but also locusts, beetles, and even flies and fleas: might these not have been present in the Ark in greater numbers than the total specified when God gave the command?  First, then, we must remind those who are troubled by this question that the words 'every creeping thing of the earth' (Genesis 6:20) are to be taken to mean that there was no need to preserve in the Ark those creatures able to live in the waters: not only such aquatic creatures as fish, but also those which swim on the surface; many birds, for example.  Again, when it is said, 'they shall be male and female' (Genesis 6:19), this is clearly to be understood in terms of the need to renew the species, and so it was not necessary for there to be in the Ark those creatures which can be generated from certain things, or from the corruption of such things, without sexual intercourse.  Or, if they were present in the Ark, as they are usually present in house, they could have been there without the fixing of a definite number.  On the other hand, a most sacred mystery was there being enacted; and it may be that so great a truth was being prefigured that it could not be properly conveyed unless al the creatures which were prevented by their nature from living in the waters were present in that fixed number.  If so, however, this was not the responsibility of a man or of men, but of God.  For Noah did not catch the animals and then put them in; he let them in as they came and entered.  And this is why it is said, 'they shall come unto thee' (Genesis 6:20): not, that is, by any act of man, but by the command of God.

Sexless creatures were not included on the Ark according to Augustine

The Animals Enter the Ark, c. 1896-1902, by James Jacques Joseph Tissot (French, 1836-1902)

We are not, however, to believe that this included those creatures who lack sex; for it was definitely prescribed that 'they shall be male and female'.  There are some animals, such as flies, which are generated from certain things without sexual intercourse, but which subsequently reproduce by means of intercourse.  There are others, such as bees, in which there are no male and female characteristics.  Again, there are creatures which, though they have sexual organs, do not produce young, such as male and female mules; and it would be a wonder if these last were included in the Ark.  Instead, it would suffice to have their parents there, that is, the species of h orse and ass.  And so too with any other animals which produce some different kind of creature by a mingling of different species.  But if such creatures had anything to contribute to the symbolic meaning of the Ark, then they were included, for such a species also has male and female.

Some are troubled regarding what food the carnivorous animals ate while on the Ark

Noah's ark. 1882. The State Russian Museum, St. Petersburg, Andrei Ryabushkin (1861–1904)

Not a few people are troubled as to the kinds of food which those animals which are thought to eat nothing but flesh could have had had in the Ark.  They wonder whether animals in excess of the prescribed number were taken on board, without transgressing the command, since their inclusion would have been compelled by the need to feed the others; or whether (and this is the more readily believable explanation) there could have been some form of nourishment apart from flesh which would have been suitable for all the animals.  For we know that many animals whose food if flesh also eat vegetables and fruit, especially figs and chestnuts.  Would it have been any wonder, therefore, if so wise and righteous man as Noah, divinely instructed as to the food appropriate for every animal, had prepared and established a stock of meatless food suitable to each kind?

Some are concerned about what the animals would have eaten on the Ark

A previously unreadable fragment of the Dead Sea Scrolls photographed by the Jet Propulsion Laboratory at NASA in the early 1990s using digital infrared technology. The fragment, translated into English, reads "he wrote the words of Noah."

What is there that we would not eat if compelled by hunger?  Again, what is there that God could not make pleasant and wholesome?  The divine power could, indeed, easily have endowed these animals with the ability to live without food at all, were it not for the fact that their eating had its part to play in completing the allegorical representation of so great a mystery.  For only a love of contention would allow anyone to suppose that the many signs contained in the historical story of the Flood do not prefigure the Church.  For the nations have already filled the Church, and clean and unclean alike are, as it were, contained in the hull of the Church's unity, until the appointed end is reached.  The meaning in this regard is so manifestly clear that we cannot lawfully doubt that the other aspects of the story have their own meanings, even thought he language is somewhat more obscure and difficult to understand.

Augustine argues that the events recorded in the account of the Flood are historical and that their symbolic meaning is to prefigure the Church

Noah. Mosaic in Basilica di San Marco, Venice, XII-XIII century. "For only a love of contention would allow anyone to suppose that the many signs contained in the historical story of the Flood do not prefigure the Church," says Augustine.

This being so, then, no one, no matter how stubborn, will dare to suppose that the story of the Flood was written without purpose; or that the events there recorded have only historical significance; or that they have no historical, but only symbolic meaning; or that their symbolism has nothing to do with the Church.  Rather, we are to believe that the writing of this account had a wise purpose; that the events recorded are historical; that they have a symbolic meaning also, and that the symbolic meaning is intended to prefigure the Church.

Source: Augustine. The City of God against the Pagans. Edited and translated by R.W. Dyson. Cambridge: Cambridge University Press, 1998, Book XV, Chapter 27, pages 689-693.