1) THE DAUGHTER WHO ACCEPTED HER KIDUSHIN IN THE CITY
QUESTION: Rav and Shmuel disagree about the law in a case in which a father, traveling on the road, accepts Kidushin on behalf of his daughter, while his daughter, in the city, accepts Kidushin for herself from a different man on the same day, and on that day she is found to be a Bogeres. Rav rules that since she is presently a Bogeres, she is presumed to have been a Bogeres at the time she accepted the Kidushin, and thus her Kidushin (and not her father's) is valid. Shmuel rules that she is Safek Mekudeshes (and she needs a Get from both men) because of the concern that she may have been a Na'arah at the time her father accepted Kidushin for her.
Rav and Shmuel seem to disagree about what the daughter's present status as a Bogeres reveals about her status earlier. Rav maintains that her present status presumably came about at the beginning of the day, and thus she was a Bogeres at the time her father accepted Kidushin for her. Shmuel maintains that her present state does not reveal anything about her state prior to the present moment.
This dispute should have been expressed in a more straightforward, simpler case. When her father accepted Kidushin for his daughter in the morning, and in the afternoon she was found to be a Bogeres, is the father's Kidushin valid or not? Why does the Gemara involve an apparently extraneous detail in this question, the fact that "she accepted Kidushin for herself in the city"?
ANSWER: TOSFOS explains why the Gemara specifically includes this detail in the case. Tosfos points out that the Gemara continues and asks a question on the view of Shmuel from the case of a Mikvah found to have less than the minimum amount of water. The Mishnah (Mikva'os 2:2) states that all items immersed in that Mikvah, even before it was found to be lacking, are Tamei retroactively (from the time its water level was last measured). This ruling contradicts the view of Shmuel who says that the previous status of an item may not be determined by its present status. The Gemara answers that in the case of the Mikvah, the doubt involves not only the Mikvah (whether it had the minimum amount of water at the time the items were immersed), but also the items that were immersed there. Before their immersion they were definitely Tamei, and now there is a doubt about whether or not they are Tahor. Since the items were Tamei until now they are assumed to be Tamei based on their "Chezkas Tum'ah" and not merely because the Mikvah is invalid at present. Shmuel stated his opinion (that the present state of an item does not determine its previous state) only for cases in which there is no pre-existing Chazakah. In cases in which there is a Chazakah which dictates the item's previous status, as well as a present factor which dictates the item's present status, Shmuel agrees with Rav that the earlier status of the item (after the time of the Chazakah and before the present status) may be determined based on the present status.
However, in the simple case of a father who accepted Kidushin for his daughter in the morning and then in the afternoon she was found to be a Bogeres, there also exists a Chazakah that the daughter until now was not married ("Chezkas Penuyah"). This Chazakah dictates that since the daughter was single until now, she presumably was still a Penuyah at the time the doubt arose (i.e. in the morning, when the doubt arose about whether the father's acceptance of Kidushin for his daughter was valid or not). In this case, even Shmuel would agree that the fact that she is now a Bogeres is valid grounds to assume that she had the same status earlier, when the doubt arose (and her father's Kidushin did not take effect). Why, then, does Shmuel argue in this case?
It must be that the daughter does not have a "Chezkas Penuyah." If, however, she certainly was not married until the doubt arose, why should she not have a "Chezkas Penuyah"? The only way she could lose her "Chezkas Penuyah" is if she was definitely married at the time the doubt arose. Therefore, the Gemara adds a new detail to the case -- the fact that the daughter accepted Kidushin for herself in the city. Since she accepted Kidushin for herself, she is definitely married regardless of the doubt about whether the father's Kidushin took effect; she was married either because of the Kidushin she accepted or because of the Kidushin her father accepted (depending on when she became a Bogeres). The Chazakah that she is a Penuyah no longer exists, and thus the dispute between Rav and Shmuel applies.
2) A "CHEZKAS HA'GUF" THAT A WOMAN WAS A "NA'ARAH"
OPINIONS: The Gemara suggests that the dispute between Rav and Shmuel, who argue about whether a person's prior status (which is in doubt) may be determined based on the person's present status, is a Machlokes Tana'im. Rebbi Yakov and Rebbi Nasan disagree about the law in the case of a person who gave away all of his property and now seeks to retract his gift. He claims that he was a Shechiv Mera (a dying man) when he gave away his property, and the gift was contingent upon his death. Since he recovered, the gift is not valid. The recipients claim that he was in good health when he gave them the gift, and thus the gift remains valid. Rebbi Yakov rules that the "Chezkas Mamon" dictates that since the property was in the possession of the owner until the time of the doubt, it still is in his possession, and the recipients of the gift must bring proof to their claim. Rebbi Nasan rules that only if the benefactor is presently a Shechiv Mera is he assumed to have been a Shechiv Mera when he gave the gift (and the recipients must bring proof to the contrary). If he is healthy now, he is assumed to have been healthy at the time he gave the gift, and the recipients prevail. Apparently, Rav follows the view of Rebbi Nasan, and Shmuel follows the view of Rebbi Yakov.
The Gemara responds that the dispute between Rebbi Yakov and Rebbi Nasan is unrelated to the dispute between Rav and Shmuel. Rav may follow the view of Rebbi Yakov since, in that case, there exists a "Chezkas Mamon" (the owner of the property owned it until now). In contrast, in the case in which Rav and Shmuel disagree there is no "Chezkas ha'Guf" that the daughter was a Na'arah.
The Rishonim disagree about why there is no "Chezkas ha'Guf" that the daughter was a Na'arah.
(a) RASHI (DH Mi Ika) explains that in the case of Rav and Shmuel, six months have passed since the daughter entered the stage of Na'arus. On the last day of those six months she is likely to became a Bogeres. Since there is a likelihood that her status will change on that day, there is no Chazakah that she is a Na'arah. A Chazakah may be established only when there is strong reason to assume that the status remains the same. Since, in this case, the girl's status naturally changes at the end of the six-month period of Na'arus, there is no Chazakah that she is still a Na'arah at that time.
(b) TOSFOS (DH Mi Ika) explains in the name of the RI that it is normal for a girl's physical changes to occur in the morning. Hence, on this day (the day the doubt arose) there is no Chazakah that she is still a Na'arah.
According to the Ri, the fact that a girl's status is inclined to change at the end of the six-month period of Na'arus is not sufficient grounds to disestablish the Chazakah. Rather, there must be a degree of "Regilus" -- a degree of regularity and frequency that the change occurs in the morning. The Chazakah that she is a Na'arah is eliminated only when her status both is likely to change on that day and it normally changes in the morning.
The SHEV SHEMAITSA (3:16) explains that the dispute between Rashi and the Ri is based on their understanding of the general concept of Chazakah (specifically, a "Chazakah d'Hashta"). Does a Chazakah work backward in time? That is, when no "Chazakah d'Me'ikara" is present (a "status-quo" Chazakah which resolves a doubt by giving the person or object the status it had until the doubt arose), can the status at the time of the doubt be determined based on the present status, with a "Chazakah d'Hashta"?
According to Rashi, there is no "Chazakah d'Me'ikara" in the case of the Gemara here. Therefore, according to Rav, the doubt is resolved in accordance with the "Chazakah d'Hashta" (since she is a Bogeres now, she is assumed to have been a Bogeres at the time of the Safek).
According to Tosfos, the Gemara here does not prove that a "Chazakah d'Hashta" may resolve a doubt. In the case of the Gemara here, there is no "Chazakah d'Me'ikara" because of "Regilus" -- it regularly happens that the girl's status changes at that time. What dictates that the girl is not a Na'arah is not the fact that now she is a Bogeres ("Chazakah d'Hashta"), but rather the fact that it is usual for a girl's status to change in the morning of the day on which the six-month period of Na'arus ends.

79b----------------------------------------79b

3) INVESTIGATING THE LINEAGE OF A WOMAN WHO COMES FROM "MEDINAS HA'YAM"
QUESTION: The Mishnah lists the circumstances under which Beis Din must investigate the lineage of a woman and her children in a case in which a man returns from abroad with a wife and children. The Mishnah states that when the man and woman first married and then went abroad and returned with children, there is no need to investigate the lineage of the woman or the children. As RASHI explains, her lineage was already confirmed at the time of her marriage.
Why should it be necessary to investigate a woman's lineage altogether?
ANSWERS:
(a) RASHI apparently understands that the Mishnah follows the view of Rebbi Meir. The Gemara earlier (76b) cites a dispute between Rebbi Meir and the Rabanan with regard to the requirement to investigate a woman's lineage before one marries her. Rebbi Meir requires such an investigation (76a), while the Rabanan do not require such an investigation because "all families are b'Chezkas Kesheros."
(b) The TOSFOS RI HA'ZAKEN asserts that the Mishnah may follow even the view of the Rabanan. He differentiates between a woman who comes from a known, recognized community and one who comes from Medinas ha'Yam, a foreign, unknown place. The Rabanan maintain that every family has a "Chezkas Kashrus" only when that family is recognized and known not to have any blemishes. A woman who comes from Medinas ha'Yam, however, comes from an unknown family and has no Chazakah of Kashrus. Hence, even the Rabanan agree that if not for the fact that the husband is believed to say that the woman is his original wife (whom he married before he left to Medinas ha'Yam), an investigation of her lineage would be necessary. The man is not believed to say that a Bedikah was already done (contrary to the view of Rashi), but only to say that his wife is from a known region, and thus no investigation is necessary. (See BEIS SHMUEL 4:3, as cited in Insights to 72:2, who records two views on this matter which seem to be the views of Rashi and Tosfos Ri ha'Zaken.)