Temurah 16 - 22
The Forgotten Laws
Three thousand laws were forgotten by Israel during the days of mourning for Moshe. When the people approached his successor, Yehoshua, and begged him to ask for these laws to be revealed again by Heaven, he replied "It (the Torah) is not in Heaven" (Devarim 30:12). At a later stage in history they approached the Prophet Shmuel with the same request and he explained his inability to do so based on the passage "These are the commands" (Vayikra 27:34) which implies that after Moshe no new permanent commands will be relayed through a prophet.
Both Yehoshua and Samuel realized that they were restrained from prophetically receiving Divine commands to add on to the ones Moshe had relayed to his people. But Yehoshua did not explain this restraint in the same way as Shmuel did. The reason, explains Maharsha, is because Yehoshua had already heard these commands from Moshe but merely forgot them. Having them restored to his memory by prophecy would therefore not come under the category of new commands. He therefore quoted the other passage which is far more comprehensive in regard to a prophet's limitations by declaring that even a clarification of a Torah command will no longer be communicated through a prophet and can only be decided upon through the wisdom of the Sages.
When Gender is a Blemish
Both the olah sacrifice and the asham sacrifice can be brought only from the male animal. But there is a difference in the status of a female animal consecrated for one of these sacrifices.
In regard to a female consecrated for an olah there is a consensus among the Sages that the animal achieves a status of personal sanctity which can only be removed by redemption if the animal develops a blemish. Only then may the money realized from its redemption be utilized for purchasing a male animal to serve as on olah.
But when it comes to a female consecrated for an asham it is the opinion of Rabbi Shimon that its ineligibility because of gender to serve as a sacrifice is in itself considered a blemish, and it may therefore be redeemed without recourse to any physical blemish.
The difference is thus explained by Rashi:
A female fowl qualifies as an olah sacrifice. We can, therefore, not view gender as an absolute blemish in regard to olah and a physical blemish is required for redemption according to all opinions. Since there is no asham sacrifice from fowl and there is never a possibility of a female serving as an asham, Rabbi Shimon considers the gender to be a sufficient blemish to qualify it for redemption.