Kilayim, Chapter 1, Mishnah 6. After dealing with seeds and trees, the Mishnah turns to animals that look similar to one another but are in fact two separate species that may not be mixed.
The pairs of animals listed by the Mishnah:
"Ha'ze'ev veha'kelev" - the wolf and the dog: two distinct animals, and even though they look alike, they may not be mixed with each other.
"Kelev hakufri veha'shu'al" - the kufri dog and the fox: the kufri dog is a dog raised in the small villages, used mainly for guarding and for hunting, and its appearance resembles a fox. Even so, this type of wild dog and the fox are two different species.
"Ha'izim veha'tzeva'im" - goats and gazelles: on the surface there are certain points of resemblance between them, and nevertheless they are considered two different species.
"Ha'ye'elim veha'rechelim" - ye'elim are a type of gazelle, and rechelim are female sheep (ewes), and they are two different species.
"Ha'sus veha'pered" - the horse and the mule: two different species.
"Ha'pered veha'chamor" - the mule is the offspring of a mixture of a donkey and a horse, and nevertheless it is considered a species distinct from the species it came from.
"Ha'chamor veha'arod" - the donkey, as opposed to the arod, which is a particular type of wild donkey.
The Mishnah concludes by ruling regarding all of these pairs: "Af al pi shedomin zeh lazeh - kilayim zeh bazeh" - even though they look similar to one another, they are considered two different species, and they may not be mixed: neither to mate them with one another nor to work with them together.
In summary: this Mishnah lists pairs of animals that bear an outward resemblance to one another - the wolf and the dog, the kufri dog and the fox, goats and gazelles, ye'elim and ewes, the horse and the mule, the mule and the donkey, and the donkey and the arod - and teaches that the similarity in their appearance does not unite them into a single species, and each of these pairs has the status of kilayim.