Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We must now consider the work of the sixth day.
Objection 1: It would seem that this work is not fittingly described. For as birds and fishes have a living soul, so also have land animals. But these animals are not themselves living souls. Therefore the words, "Let the earth bring forth the living creature," should rather have been, "Let the earth bring forth the living four-footed creatures."
Objection 2: Further, a genus ought not to be opposed to its species. But beasts and cattle are quadrupeds. Therefore quadrupeds ought not to be enumerated as a class with beasts and cattle.
Objection 3: Further, as animals belong to a determinate genus and species, so also does man. But in the making of man nothing is said of his genus and species, and therefore nothing ought to have been said about them in the production of other animals, whereas it is said "according to its genus" and "in its species."
Objection 4: Further, land animals are more like man, whom God is recorded to have blessed, than are birds and fishes. But as birds and fishes are said to be blessed, this should have been said, with much more reason, of the other animals as well.
Objection 5: Further, certain animals are generated from putrefaction, which is a kind of corruption. But corruption is repugnant to the first founding of the world. Therefore such animals should not have been produced at that time.
Objection 6: Further, certain animals are poisonous, and injurious to man. But there ought to have been nothing injurious to man before man sinned. Therefore such animals ought not to have been made by God at all, since He is the Author of good; or at least not until man had sinned.
On the contrary, Suffices the authority of Scripture.
I answer that, As on the fifth day the intermediate body, namely, the water, is adorned, and thus that day corresponds to the second day; so the sixth day, on which the lowest body, or the earth, is adorned by the production of land animals, corresponds to the third day. Hence the earth is mentioned in both places. And here again Augustine says (Gen. ad lit. v) that the production was potential, and other holy writers that it was actual.
Reply to Objection 1: The different grades of life which are found in different living creatures can be discovered from the various ways in which Scripture speaks of them, as Basil says (Hom. viii in Hexaem.). The life of plants, for instance, is very imperfect and difficult to discern, and hence, in speaking of their production, nothing is said of their life, but only their generation is mentioned, since only in generation is a vital act observed in them. For the powers of nutrition and growth are subordinate to the generative life, as will be shown later on (Q[78], A[2]). But amongst animals, those that live on land are, generally speaking, more perfect than birds and fishes, not because the fish is devoid of memory, as Basil upholds (Hom. viii in Hexaem.) and Augustine rejects (Gen. ad lit. iii), but because their limbs are more distinct and their generation of a higher order, (yet some imperfect animals, such as bees and ants, are more intelligent in certain ways). Scripture, therefore, does not call fishes "living creatures," but "creeping creatures having life"; whereas it does call land animals "living creatures" on account of their more perfect life, and seems to imply that fishes are merely bodies having in them something of a soul, whilst land animals, from the higher perfection of their life, are, as it were, living souls with bodies subject to them. But the life of man, as being the most perfect grade, is not said to be produced, like the life of other animals, by earth or water, but immediately by God.
Reply to Objection 2: By "cattle," domestic animals are signified, which in any way are of service to man: but by "beasts," wild animals such as bears and lions are designated. By "creeping things" those animals are meant which either have no feet and cannot rise from the earth, as serpents, or those whose feet are too short to life them far from the ground, as the lizard and tortoise. But since certain animals, as deer and goats, seem to fall under none of these classes, the word "quadrupeds" is added. Or perhaps the word "quadruped" is used first as being the genus, to which the others are added as species, for even some reptiles, such as lizards and tortoises, are four-footed.
Reply to Objection 3: In other animals, and in plants, mention is made of genus and species, to denote the generation of like from like. But it was unnecessary to do so in the case of man, as what had already been said of other creatures might be understood of him. Again, animals and plants may be said to be produced according to their kinds, to signify their remoteness from the Divine image and likeness, whereas man is said to be made "to the image and likeness of God."
Reply to Objection 4: The blessing of God gives power to multiply by generation, and, having been mentioned in the preceding account of the making of birds and fishes, could be understood of the beasts of the earth, without requiring to be repeated. The blessing, however, is repeated in the case of man, since in him generation of children has a special relation to the number of the elect [*Cf. Augustine, Gen. ad lit. iii, 12], and to prevent anyone from saying that there was any sin whatever in the act of begetting children. As to plants, since they experience neither desire of propagation, nor sensation in generating, they are deemed unworthy of a formal blessing.
Reply to Objection 5: Since the generation of one thing is the corruption of another, it was not incompatible with the first formation of things, that from the corruption of the less perfect the more perfect should be generated. Hence animals generated from the corruption of inanimate things, or of plants, may have been generated then. But those generated from corruption of animals could not have been produced then otherwise than potentially.
Reply to Objection 6: In the words of Augustine (Super. Gen. contr. Manich. i): "If an unskilled person enters the workshop of an artificer he sees in it many appliances of which he does not understand the use, and which, if he is a foolish fellow, he considers unnecessary. Moreover, should he carelessly fall into the fire, or wound himself with a sharp-edged tool, he is under the impression that many of the things there are hurtful; whereas the craftsman, knowing their use, laughs at his folly. And thus some people presume to find fault with many things in this world, through not seeing the reasons for their existence. For though not required for the furnishing of our house, these things are necessary for the perfection of the universe." And, since man before he sinned would have used the things of this world conformably to the order designed, poisonous animals would not have injured him.