the Lord, in order to inculcate pity to the Jewish people, who were prone to cruelty, wished them to practice pity even with regard to dumb animals, and forbade them to do certain things savoring of cruelty to animals.  Hence He prohibited them to “boil a kid in the milk of its dam”; and to “muzzle the ox that treadeth out the corn”; and to slay “the dam with her young.”  It may, nevertheless, be also said that these prohibitions were made in hatred of idolatry.  For the Egyptians held it to be wicked to allow the ox to eat of the grain while threshing the corn.  Moreover certain sorcerers were wont to ensnare the mother bird with her young during incubation, and to employ them for the purpose of securing fruitfulness and good luck in bringing up children:  also because it was held to be a good omen to find the mother sitting on her young.

As to the mingling of animals of divers species, the literal reason may have been threefold.  The first was to show detestation for the idolatry of the Egyptians, who employed various mixtures in worshipping the planets, which produce various effects, and on various kinds of things according to their various conjunctions.  The second reason was in condemnation of unnatural sins.  The third reason was the entire removal of all occasions of concupiscence.  Because animals of different species do not easily breed, unless this be brought about by man; and movements of lust are aroused by seeing such things.  Wherefore in the Jewish traditions we find it prescribed as stated by Rabbi Moses that men shall turn away their eyes from such sights.

The figurative reason for these things is that the necessities of life should not be withdrawn from the ox that treadeth the corn, i.e. from the preacher bearing the sheaves of doctrine, as the Apostle states (1 Cor. 9:4, seqq.).  Again, we should not take the dam with her young:  because in certain things we have to keep the spiritual senses, i.e. the offspring, and set aside the observance of the letter, i.e. the mother, for instance, in all the ceremonies of the Law.  It is also forbidden that a beast of burden, i.e. any of the common people, should be allowed to engender, i.e. to have any connection, with animals of another kind, i.e. with Gentiles or Jews.

Reply Obj. 9:  All these minglings were forbidden in agriculture; literally, in detestation of idolatry.  For the Egyptians in worshipping the stars employed various combinations of seeds, animals and garments, in order to represent the various connections of the stars.  Or else all these minglings were forbidden in detestation of the unnatural vice.

They have, however, a figurative reason.  For the prohibition:  “Thou shalt not sow thy field with different seeds,” is to be understood, in the spiritual sense, of the prohibition to sow strange doctrine in the Church, which is a spiritual vineyard.  Likewise “the field,” i.e. the Church, must not be sown “with different seeds,” i.e. with Catholic and heretical doctrines.  Neither is it allowed to plough “with an ox and an ass together”; thus a fool should not accompany a wise man in preaching, for one would hinder the other.