1) BREEDING MULES
QUESTION: Rebbi Yehudah says that a male offspring of a horse and a donkey may be bred with a female offspring of a horse and a donkey. The Gemara explains that we do not consider this to be crossbreeding the maternal side of the male offspring (the horse) with the paternal side of the female offspring (the donkey), which would be prohibited as Kil'ayim.
However, another Halachah in a similar case seems to contradict this. The Gemara in Bava Basra (13a) discusses a person who is half-Eved and half-free ("Chatzyo Eved v'Chatzyo Ben Chorin"; such a person was originally an Eved owned by two partners, and then one of the owners freed his share of the Eved). Such a person is not permitted to marry any woman, and not even a woman who is herself half-Eved and half-free, because the free half of him will be having relations with the slave half of her, which is prohibited. Similarly, in the case of the Gemara here, the half of the mule that came from the horse is breeding with the half of the other animal that came from the donkey, which should be forbidden because of Kil'ayim!
ANSWER: RASHI (DH Ka Mashma Lan) writes that in this case, each offspring of the horse and donkey is not considered to be a hybrid comprised of an element of horse and an element of donkey. Rather, the concept of "Mevalbel Zar'ei" (see 69a) -- the mixing of the maternal side (horse) and the paternal side (donkey) -- results in an entirely new species, and not in a half-horse, half-donkey. Although it is similar in some ways to a horse and similar in other ways to a donkey, it is a distinct, separate species.
TOSFOS (DH Mahu) explains this further. When the Torah forbids Kil'ayim, it forbids only crossbreeding two separate species. Two mules that were produced from a horse and a donkey are considered to be a single species, and, therefore, one is permitted to breed a mule with another mule. Two species are not being mixed together.
The RAMBAN cites the Gemara in Rosh Hashanah (29a) that says that a half-freed slave cannot blow the Shofar on Rosh Hashanah, even to fulfill the Mitzvah for himself, because the half of him that is an Eved (which is exempt from the Mitzvah) cannot blow the Shofar for the half of him that is free and obligated to perform the Mitzvah. Why is that case different from the case of breeding two mules? The Ramban answers, like Rashi and Tosfos, that when the parenthood of the animal is "mixed in" and the side of the horse and the side of the donkey are no longer recognizable, there is no Isur of crossbreeding. In contrast, in the case of a half-Eved, half-free person, the two halves never "mix" with each other, but they remain two distinct parts.
The Ramban adds that this is the reason why one is permitted to ride on, or plow with, a mule (see also Tosfos to Bava Basra 13a, DH Kofin). We do not say that one who rides on a mule is riding on a horse and a donkey at the same time, transgressing the prohibition of working with two different types of animals together. Since the horse-side has blended with the donkey-side, the two sides are no longer considered separate elements but have formed a new type of animal.
Accordingly, the Ramban writes that the Halachah does not follow the view of Isi in the Yerushalmi (Kil'ayim 18:2), who rules that one is not permitted to ride a mule. Isi presents a Kal va'Chomer to support his ruling: One is permitted to wear a garment of wool and a garment of linen, one on top of the other, but one is forbidden to wear a garment of wool and linen mixed together. Consequently, since one is forbidden to lead two animals of two different species together, one certainly is forbidden to lead an animal that itself is comprised of a horse and a donkey. Isi asserts that the mule that David ha'Melech said that his son should ride (Melachim I 1:39) was a special animal created during the six days of Creation and was not born from a horse and donkey, because otherwise it would have been forbidden for Shlomo to ride on it.
The Halachah does not follow the view of Isi. Rather, the Halachah permits riding the animal because the horse-side and donkey-side mix together and lose their separate identities, merging into a new species. In contrast, in a garment containing wool and linen, each material retains its separate identity after being sewn into the garment. (D. BLOOM)
2) RELYING ON "SIMANIM"
QUESTION: The Gemara relates that Rebbi Aba said to his attendant, "If you fasten mules to my wagon, check that they are similar to each other" in the appearance of their ears and tails (RASHI DH d'Damyan; see also SHULCHAN ARUCH YD 297:9, who adds that the voices of the mules must also be similar). This will prove that they are not the offspring of two different species of animal and there is no Isur in using them together to pull the wagon.
The Gemara says that since Rebbi Aba relied on the Simanim of mules to prove that they were of the same species and to permit working with them together, this implies that "Simanin d'Oraisa" -- one may rely on Simanim (external signs) to determine a Halachic practice, even when an Isur d'Oraisa is involved.
However, the Gemara in Bava Metzia asks whether the ability to rely on Simanim is mid'Rabanan or mid'Oraisa, and it reaches no conclusion in the matter. Moreover, the Gemara earlier in Bava Metzia (18b) cites Rav Ashi who was in doubt about whether Simanim are mid'Oraisa or not. Why, then, does the Gemara here conclude unequivocally that Simanim are mid'Oraisa?
(a) RASHI (DH v'Simanim) implies that the Gemara here indeed is answering the question that it asked in Bava Metzia about whether Simanim are mid'Oraisa or mid'Rabanan, and it is saying that Simanim are mid'Oraisa.
(b) The CHIDUSHEI HA'RAN cites the RAMBAN who disagrees with Rashi and maintains that the case of Simanim mentioned here is not comparable to the case of Simanim discussed in Bava Metzia. The Gemara there discusses relying on Simanim for the purpose of returning lost objects to their owners. It is possible that Simanim are not reliable in cases of lost objects, because there is a concern that the claimant might not be the real owner and merely might have seen the object once or is guessing about its Simanim. In contrast, the Simanim used to determine the species of mules are much more reliable, because the Chachamim possessed a tradition that these Simanim may be used to determine the identity of the species. The case of the Gemara here is more similar to the case of the Gemara earlier (64a) which concludes that one may not rely on Simanim -- different natural features -- to determine whether or not eggs came from a Kosher bird. In that case, the Chachamim did not possess a tradition about the Simanim of Kosher eggs.
The dispute between Rashi and the Ramban seems to be reflected in a dispute between later authorities. The KETZOS HA'CHOSHEN (CM 259:2) cites the SHULCHAN ARUCH (EH 17:24) who rules that we may not rely on Simanim of a corpse in order to determine the identity of the dead man and permit his widow to remarry. The Ketzos ha'Choshen cites other authorities who question this ruling on the basis of the Gemara here in Chulin. He cites RABEINU OZER who answers that in the Gemara here, there are two signs (the ears and tail) that must be similar in order to permit working with the two animals together. Similarly, if the corpse can be identified on the basis of two signs, then we may rely on those Simanim to permit the widow to remarry. Rabeinu Ozer apparently sides with Rashi who maintains that we compare the case of Simanim here with the case of Simanim in Bava Metzia. The Ketzos ha'Choshen, on the other hand, apparently agrees with the Ramban, who maintains that the Simanim of the mules are natural signs on which the Chachamim said we may rely, while we may not rely on the Simanim of a corpse, because perhaps the widow saw the body before, or she is merely guessing.
(c) The MAHARATZ CHAYOS cites the NODA B'YEHUDAH who says that we may always rely on Simanim except when they oppose an existing Chazakah. Consequently, a woman who was known to be married until now ("Chezkas Eshes Ish") cannot be permitted to remarry based on signs on the corpse (or based on signs on a Get that was found). In contrast, the mules have no Chazakah of being of different species, and, therefore, we may rely on Simanim to identify them as being of one species. (See also Maharatz Chayos to 96a.) (D. BLOOM)