Genesis 1:22

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.

I blame Matter with a like censure, because out of it, 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.

What, then, is to be said about these theories? This that those who would have it 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 connexion between them which blends and confuses hopelessly all the marks by which one could be distinguished from another.

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.