Ius naturale est, quod natura omnia animalia docuit: nam ius istud non humani generis proprium, sed omnium animalium, quae in terra, quae in mari nascuntur, avium quoque commune est.  Hinc descendit maris atque feminae coniunctio, quam nos matrimonium appellamus, hinc liberorum procreatio, hinc educatio. (Ius naturale is that which nature has taught all animals.  This ius does not just pertain to the human race but is common to all animals, those born on the land and in the sea, but also birds.  From this ius comes the union of man and woman, which we call marriage, the procreation and raising of children.)

De ritu nuptiarum

Justinian's Digest, Firenze, Biblioteca Laurenziana, Sine numero  (ca. 600 AD)