Objection 1. It would seem that this work is not fittingly described. For the waters produce that which the power of water suffices to produce. But the power of water does not suffice for the production of every kind of fishes and birds since we find that many of them are generated from seed. Therefore the words, "Let the waters bring forth the creeping creature having life, and the fowl that may fly over the earth," do not fittingly describe this work.
Objection 2. Further, fishes and birds are not produced from water only, but earth seems to predominate over water in their composition, as is shown by the fact that their bodies tend naturally to the earth and rest upon it. It is not, then, fittingly that fishes and birds are produced from water.
Objection 3. Further, fishes move in the waters, and birds in the air. If, then, fishes are produced from the waters, birds ought to be produced from the air, and not from the waters.
Objection 4. Further, not all fishes creep through the waters, for some, as seals, have feet and walk on land. Therefore the production of fishes is not sufficiently described by the words, "Let the waters bring forth the creeping creature having life."
Objection 5. Further, land animals are more perfect than birds and fishes which appears from the fact that they have more distinct limbs, and generation of a higher order. For they bring forth living beings, whereas birds and fishes bring forth eggs. But the more perfect has precedence in the order of nature. Therefore fishes and birds ought not to have been produced on the fifth day, before land animals.
On the contrary, Suffices the authority of Scripture.
I answer that, As said above, (70, 1), the order of the work of adornment corresponds to the order of the work of distinction. Hence, as among the three days assigned to the work of distinction, the middle, or second, day is devoted to the work of distinction of water, which is the intermediate body, so in the three days of the work of adornment, the middle day, which is the fifth, is assigned to the adornment of the intermediate body, by the production of birds and fishes. As, then, Moses makes mention of the lights and the light on the fourth day, to show that the fourth day corresponds to the first day on which he had said that the light was made, so on this fifth day he mentions the waters and the firmament of heaven to show that the fifth day corresponds to the second. It must, however, be observed that Augustine differs from other writers in his opinion about the production of fishes and birds, as he differs about the production of plants. For while others say that fishes and birds were produced on the fifth day actually, he holds that the nature of the waters produced them on that day potentially.
Reply to Objection 1. It was laid down by Avicenna that animals of all kinds can be generated by various minglings of the elements, and naturally, without any kind of seed. This, however, seems repugnant to the fact that nature produces its effects by determinate means, and consequently, those things that are naturally generated from seed cannot be generated naturally in any other way. It ought, then, rather to be said that in the natural generation of all animals that are generated from seed, the active principle lies in the formative power of the seed, but that in the case of animals generated from putrefaction, the formative power of is the influence of the heavenly bodies. The material principle, however, in the generation of either kind of animals, is either some element, or something compounded of the elements. But at the first beginning of the world the active principle was the Word of God, which produced animals from material elements, either in act, as some holy writers say, or virtually, as Augustine teaches. Not as though the power possessed by water or earth of producing all animals resides in the earth and the water themselves, as Avicenna held, but in the power originally given to the elements of producing them from elemental matter by the power of seed or the influence of the stars.
Reply to Objection 2. The bodies of birds and fishes may be considered from two points of view. If considered in themselves, it will be evident that the earthly element must predominate, since the element that is least active, namely, the earth, must be the most abundant in quantity in order that the mingling may be duly tempered in the body of the animal. But if considered as by nature constituted to move with certain specific motions, thus they have some special affinity with the bodies in which they move; and hence the words in which their generation is described.
Reply to Objection 3. The air, as not being so apparent to the senses, is not enumerated by itself, but with other things: partly with the water, because the lower region of the air is thickened by watery exhalations; partly with the heaven as to the higher region. But birds move in the lower part of the air, and so are said to fly "beneath the firmament," even if the firmament be taken to mean the region of clouds. Hence the production of birds is ascribed to the water.
Reply to Objection 4. Nature passes from one extreme to another through the medium; and therefore there are creatures of intermediate type between the animals of the air and those of the water, having something in common with both; and they are reckoned as belonging to that class to which they are most allied, through the characters possessed in common with that class, rather than with the other. But in order to include among fishes all such intermediate forms as have special characters like to theirs, the words, "Let the waters bring forth the creeping creature having life," are followed by these: "God created great whales," etc.
Reply to Objection 5. The order in which the production of these animals is given has reference to the order of those bodies which they are set to adorn, rather than to the superiority of the animals themselves. Moreover, in generation also the more perfect is reached through the less perfect.
Source: Summa Theologica (New Advent)