Genesis 1:22

Out of Matter, evil though it be—nay, very evil—good things have been created, nay, "very good" ones: "And God saw that they were good, and God blessed them" Genesis 1:21-22 —because, of course, of their very great goodness; certainly not because they were evil, or very evil. Change is therefore admissible in Matter; and this being the case, it has lost its condition of eternity; in short, its beauty is decayed in death. [Against Hermagenes 12]

Those who would contend that the soul migrates into natures divergent from each other seem to me to obliterate all natural distinctions—to blend and confuse together in every possible respect the rational, the irrational, the sentient and the insensate; if, that is, all these are to pass into each other with no distinct natural order secluding them from mutual transition. To say that one and the same soul, on account of a particular environment of body, is at one time a rational and intellectual soul, and that then it is caverned along with the reptiles, or herds with the birds, or is a beast of burden, or a carnivorous one, or swims in the deep; or even drops down to an insensate thing so as to strike out roots or become a complete tree, producing buds on branches, and from those buds a flower, or a thorn, or a fruit edible or noxious—to say this is nothing short of making all things the same and believing that one single nature runs through all beings; that there is a connection between them which blends and confuses hopelessly all the marks by which one could be distinguished from another.

We learn from Scripture in the account of the first creation that first the earth brought forth “the green herb” (as the narrative says), and then from this plant seed was yielded, from which, when it was shed on the ground, the same form of the original plant again sprang up. The apostle, it is to be observed, declares that this very same thing happens in the resurrection also. And so we learn from him the fact not only that our humanity will be then changed into something nobler but also that what we have therein to expect is nothing else than that which was at the beginning.

Seeds of one kind cannot be changed into another kind of plant nor bring forth produce differing from its own seeds, so that men should spring from serpents and flesh from teeth. How much more, indeed, is it to be believed that whatever has been sown rises again in its own nature and that crops do not differ from their seed, that soft things do not spring from hard nor hard from soft, nor is poison changed into blood, but that flesh is restored from flesh, bone from bone, blood from blood, the humors of the body from humors.

What pure and untarnished generations follow without intermingling one after another, so that a thymallus produces a thymallus; a sea-wolf, a sea-wolf. The sea-scorpion, too, preserves unstained its marriage bed.… Fish, therefore, know nothing of union with alien species. They do not have unnatural betrothals such as are designedly brought about between animals of two different species as, for instance, the donkey and the mare, or again the female donkey and the horse, both being examples of unnatural union. Certainly there are cases in which nature suffers more in the nature of defilement rather than that of injury to the individual. Man as an abettor of hybrid barrenness is responsible for this. He considers a mongrel animal more valuable than one of a genuine species. You mix together alien species and you mingle diverse seeds.

God wanted the blessing to have the power of fecundity, which is revealed in the succession of offspring. Thus, though the animals were made weak and mortal, they might by that blessing preserve their kind by giving birth.

And God saw that it was good, and He blessed them, saying: Be fruitful and multiply, and fill the waters of the seas, and let birds multiply on the earth. And there was evening and there was morning, a fifth day. When He said, be fruitful and multiply, and fill the waters of the seas, it pertains to both kinds of creatures made from the waters, that is, both to fish and to birds, because just as all fish can live only in the waters, so too there are many birds which, although they sometimes rest on the lands and propagate their offspring, not only feed from the land but also from the sea, and prefer to inhabit marine dwellings rather than terrestrial ones. But when He adds, and let birds multiply on the earth, it pertains to both kinds of birds, that is, both those that come from the waters and those that feed from the earth, because indeed even those birds which do not know how to live without waters often hide much of the time under the depths of the waters like fish, occasionally tend to come out onto the lands, especially when they are breeding and nurturing their young.

he blessed them: The blessing of God gives power to multiply by generation, and, 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.
Before sin matrimony was instituted by God, when He fashioned a helpmate for man out of his rib, and said to them: "Increase and multiply." And although this was said also to the other animals, it was not to be fulfilled by them in the same way as by men. As to Adam's words, he uttered them inspired by God to understand that the institution of marriage was from God.