Sefer HaChinuch ספר החינוך

To not mate a beast, a species with not its species: To not mate a beast [in a] forbidden mixture — meaning to say that we not graft a male with any species of [domesticated] beast or [wild] animal which is not its species — as it is stated (Leviticus 19:19), “your beast shall you not mate [in a] forbidden mixture.” And in its explanation, they, may their memory be blessed, said (Bava Metzia 91a) that the obligation is from when he inserts like “the brush into the tube” — and then is he lashed.

It is from the roots of the commandment that God, blessed be He, created His world with wisdom, with understanding and with knowledge, and [so] He made and formed all of the forms according to that which it was fitting for its matter to be, to be designed [according to] the design of the world — and blessed is He that knows [this]. And this is what is stated about the story of creation (Genesis 1:31), “And God saw everything that He had done, and behold, it was very good.” And His seeing, blessed be He, is His knowledge and contemplation about things; as He, blessed be He, does not need seeing things with the eye after the act, due to His great level; since everything is revealed and known and apparent in front of Him before the act, [just] like after the act. But the Torah speaks to people with words directed to them, and it calls things by the name of things that are aimed at them, as it is only possible to speak with a creature with what is known to him — as who can understand what he does not have the power to understand? And about what is similar to this, they, may their memory be blessed, said (Mekhilta DeRabbi Yishmael 19:18:2), [it is] in order to break [to assuage] the ear to that which it can hear. And in that God knew that everything He made was designed perfectly for its matter that it needed in His world, He commanded to each and every species to make its “fruit” for its species — as it is written in the Order of Bereshit — and that the species not mix, lest it will take away from their perfection and He [therefore] not command His blessing upon them. And according to what it seems in our thoughts, it is from this root that we were prevented from mating beasts [in a] forbidden mixture. And likewise from this reason were we warned about this, combined with another reason that we already wrote about species of seeds and trees (Sefer HaChinukh 62).

From the laws of the commandment is that which they, may their memory be blessed, said (Bava Kamma 54b) that it is not specifically [domesticated] beasts that are [under] this prohibition, but even a [wild] animal or a bird — so long as he mates them with what is not their species, he is lashed for them. And even if he mates a [domesticated] beast or a [wild] animal with an animal in the sea, he is lashed. And it is one, whether it is a [domesticated] beast, a [wild] animal or a bird of his, or [one] of his fellow — he is lashed for them. And that which they said (Bava Metzia 91b) that it is permissible to bring in two species into one pen; and if they mate, he is not required to separate them. And that which they said (Bava Metzia 90a) that it is forbidden for an Israelite to mate his beast [in a] forbidden mixture, by way of a gentile. And so [too,] that which they said that one who transgressed and grafted [it] with what is not its species, what is born from them is permitted in benefit and in eating — so long as they are both pure animals. And anything that is two species is included in this prohibition, even if they are similar to one another. And anything that is one species, even though one lives in the wild and the other [with man], is permissible. But the domesticated goose and the wild goose are not [included] in this dispensation, since they are two species, as the testicles of the domesticated are internal and the testicles of the wild are external. And a koi is a forbidden mixture with a [domesticated] beast and with a [wild] animal; but we do not administer lashes for it, since [its status] is a doubt.

And regarding this prohibition, everything follows the mother, such that we are not concerned about the seed of the father at all, according to that which is apparent from the decision of Rabbi Alfasi, may his memory be blessed. But there are other commentators (Tosafot s.v. aeyil on Chullin 79a) that decided that since they were in doubt about this matter in the Gemara — whether we are concerned about the seed of the father or not concerned — we always go towards the stringent about it. And regarding “it and its child,” we are concerned about the seed of the father, and [so] we do not slaughter the father with the child if we know [who] he [is]. But concerning mating, plowing and leading one with the other, we are not concerned about the seed of the father — to consider him [as] one species with the son, so long as the mother is of a different species. This is what comes out of the discussion with the good commentary in the chapter [entitled] Oto ve’et Beno (Chullin 79a). And there in the Gemara, they mentioned for us the signs [with which] to recognize the species of mules: that are like the mother with the ears, the tail and the voice. And there is no doubt that in that the species of the mother of mules is [all] the same, the species of their fathers is also the same — this is known to all men. And the rest of the details of the commandment are in Tractate Kilayim. (See Tur, Yoreh Deah 297.)

And it is practiced in every place and at all times by males and females. And one who transgresses it and mates forbidden mixtures — and that is when he inserts like “the brush into the tube” — is lashed from Torah writ. But if he [only] puts this one on that one or helps them with the voice, we strike him with lashes of rebellion.