Kilayim, Chapter 8, Mishnah 5. In the previous Mishnah we learned that a mule whose mother was a mare and a mule whose mother was a she-donkey may not be driven together. Building on this halachah, the Mishnah states:
"HaProtiyos asuros" - protiyos are mules whose mothers we have no way of identifying, whether the mother was a mare or a she-donkey. This is simply not something that can be known; all we know is that they came from a mixture. Since it is possible that one came from a donkey mother and the other from a horse mother, they are forbidden together.
"VehaRamach mutar" - two different mules whose mothers are of the same species, as was explained in the previous Mishnah, are permitted.
Adnei haSadeh:
From here the Mishnah moves on to teach us about other types of creatures, and the most fascinating of them: "VeAdnei haSadeh chayah". The novelty here is this: since the adnei hasadeh is a primate, a creature that somewhat resembles a human being, one might have thought that it would be treated to some degree by the laws of a human being, as will be explained shortly. The Tanna Kamma establishes that this is not so: it is a chayah, a creature like any other.
The Yerushalmi relates astonishing things about this species. Such a species is unknown to us today, but in fact it was an ape-like creature that was attached to the ground by a kind of cord-like structure. It was so fierce that people could not come near it, and the only way to kill it was by severing its umbilical cord from the ground. We are not familiar with anything like this, but if it existed, this is its halachah: the law of a chayah.
"Rabbi Yosi omer: metamei b'ohel k'adam" - Rabbi Yosi disagrees and holds that it resembles a human being to a certain degree in halachah: it resembles a human being enough that when it dies it transmits tumah to anyone who was with it under the same roof.
It is worth noting that the question of whether the tumah of an ohel applies to a non-Jew is itself a dispute among the Tanna'im. The Shulchan Aruch writes that one should be stringent, but according to the letter of the law the tumah of an ohel does not apply to a gentile who has died. Tumah through contact does apply, so one who touches the body becomes tamei, but remaining under the same roof is a stringency that a kohen should be careful about where possible. In any case, this creature is certainly not included in Israel, and it therefore stands to reason that Rabbi Yosi's position follows the opinion that a gentile too conveys the tumah of an ohel.
The Kupad and the Choldas haSena'im:
The next creature mentioned in the Mishnah is the "haKupad" - the porcupine, and also the "choldas haSena'im" - a type of weasel that tends to move about among thorn bushes, and it too somewhat resembles a porcupine (its exact form is not clear to us). The Tanna Kamma establishes that both of them, the kupad and the choldas hasena'im, have the status of a chayah.
It appears that the Tanna Kamma means to reject the suggestion that these are a sheretz. And the difference between a sheretz and an animal or a chayah is significant in halachah:
An animal or a chayah: if it dies, its neveilah, its carcass, transmits tumah.
A sheretz: in this category of crawling creature, only eight species of sheratzim are tamei, and even those eight are not tamei in the same manner and not with the same severe tumah that we call tumas neveilah.
Therefore the Tanna Kamma says that the kupad and the choldas hasena'im are not included among the eight sheratzim; their status is that of a chayah, a creature that will be tamei with tumas neveilah if it dies.
However, the Mishnah continues and says specifically about the "choldas haSena'im", and not about the kupad: "Rabbi Yosi omer: Beis Shammai omrim, metamei k'zayis b'masa". This is a rare case in which the opinion of Beis Shammai is brought without an opinion of Beis Hillel being brought alongside it. According to them, the choldas hasena'im transmits tumah with a k'zayis through carrying: one who carries a piece of the dead creature, even without touching it, becomes tamei through the carrying, when it is the size of a k'zayis.
This halachah is the halachah of neveilah: the measure for tumas neveilah is a k'zayis, and it also transmits tumah through carrying. This is unlike a sheretz, whose measure is the size of a lentil, far smaller, but which does not transmit tumah through carrying, only through contact.
And from here: since they were in doubt whether its status is that of an animal or chayah on the one hand, or perhaps it is a sheretz and one of the eight sheratzim - perhaps it is the "choled" mentioned in Parashas Shemini as one of the eight sheratzim - they were stringent in both directions: at the measure of a k'zayis it transmits tumah even through carrying, like the law of neveilah; and at the smaller measure, the size of a lentil, which is the measure of a sheretz, it is tamei, but only through contact, like the law of a sheretz.
In summary: in this Mishnah we learned that protiyos, whose mothers' species cannot be identified, are forbidden because of the doubt, whereas the ramach is permitted; that adnei hasadeh, the ape-like creature attached to the ground, has the status of a chayah according to the Tanna Kamma, while according to Rabbi Yosi it transmits tumah in an ohel like a human being; and that the kupad and the choldas hasena'im have the status of a chayah and not of a sheretz. We also examined the opinion of Beis Shammai regarding the choldas hasena'im, that because of the doubt whether its status is that of a neveilah or of a sheretz they were stringent in both directions: a k'zayis through carrying like a neveilah, and the size of a lentil through contact like a sheretz.