He prohibited us from breeding forbidden mixtures of animals. And that is His, may He be exalted, saying, "do not cross-breed your animals with different species" (Leviticus 19:19). And each time he breeds [them], he is lashed - on condition that he breeds them with his hand - the male organ into the female, like a brush into a tube. And in the explanation, they said (Bava Metzia 91a), "With adulterers and forbidden mixtures - from when he inserts [it] like a brush into a tube; and then he is lashed." And the regulations of this commandment have already been explained in Chapter 8 of Kilayim. (See Parashat Kedoshim; Mishneh Torah, Diverse Species 1.)