Why did Moshe teach Yisrael about the Tahor species via Aharon and his sons?
Rashi: It was a reward for remaining silent and accepting Hashem's decree with love when Nadav and Avihu died. 1
Ramban and Moshav Zekenim: Because most of the issues regarding the Kashrus of animals concern the Kohanim in particular. They had to beware against touching Tum'ah, since they needed to come to the Beis-ha'Mikdash (to bring Korbanos) and to eat Kodshei Kodshim be'Taharah. Also if a Yisrael transgressed be'Shogeg, he had to bring a Korban - which the Kohanim would then sacrifice. Moreover, they needed to teach the people the difference between Tamei and Tahor, with regard to eating and the Dinim of Tum'ah, so that they should not render Tamei the Mishkan and its Kodshim. 2
Refer also to 10:8:1:1. And the reason that Hashem spoke to Aharon twice and to Ezlazar and Isamar only one is because one cannot compare the pain of a brother who losea sibking to that of a father who loses a son.
Ramban: Indeed, this is why the Torah inserts the current Parshah in Seifer Vayikra.
Why does the Torah write "Zos ha'Chayah" and not 'Zos ha'Behemah'?
Rashi: To hint at the fact that Hashem separated Yisrael from Tum'ah and issued them with Mitzvos, 1 because, since they cleave to Hashem, they deserve to live.
Moshav Zekenim (citing Chulin, 42a): To preclude Tereifos, which cannot live. Refer to 11:2:3:2. 2
Which He did not do with the Nochrim. This can be compared to a doctor who came to visit two patients. When he instructed the family of one of them to allow him to eat whatever he fancied, and the other he placed upon a strict diet, he explained that the former was doomed to die, whereas the latter was able to live, and by what he ate, he would fully recover (Rashi, citing Midrash Tanchuma).
See Ba'al ha'Turim.
What are the implications of the word "Zos"?
Rashi: It implies that Moshe took each and every species of animal, 1 fish, bird and vermin, 2 and showed it to Yisrael, 3 saying 'This you may eat; this you may not eat!' 4
Moshav Zekenim (from Chulin, 42a): The one who holds that T'reifos can live explains 'these Chayos you may eat, and there is another species that you may not eat - a Tereifah - even though it lives. 5
Chulin, 32a: Implying that there animals of the above species that cannot live, it teaches us that a T?reifah (an animal with one of many major blemishes) cannot live. 6
See Oznayim la'Torah DH 'Zos ha'Torah' #1, who explains as to why this was necessary, bearing in mind that the Torah itself gives the Simanim of Taharah. See also DH 'Zos ha'Beheimah' #2.
See Pasuk 9, 13 & 16, respectively (Rashi).
The Ramban (Bereishis 6:19) says that there are a multitude of species, and without a miracle, ten arks (with a total volume of over three million cubic Amos) would not have contained them and their food for the year of the Flood. And every [of the thousands of] land species has a counterpart in the sea, except for a weasel (Chulin 127a)! It would take a very long time to catch every species! Why should the hunters miss learning from Moshe this entire time?! We must say that miraculously, they caught everything quickly; even so, Moshe sent hunters to minimize the miracle. Or, R. Yehudah ha'Chasid holds that Moshe showed Yisrael only the Sheratzim and the Tamei birds. We know that only these are Temei'im, but there are Simanim for animals and fish. (PF)
Because this is always what the words "Zeh" and "Zos" imply (See for example, Sh'mos 12:2).
Chulin 42a: A fact which, in fact, we learn from here .
See Torah Temimah, note 2.
Why does the Pasuk write both "Zos ha'Chayah" and "mi'Kol ha'Beheimah"?
Rashi: To teach us that "Chayah" always incorporates 'Beheimah'. 1
Sifra: mi'Kol ha'Beheimah" includes Kil'ayim (a cross between two different species of animal) in the Heter Achilah.
See Torah Temimah, note 3.
Why does the Pasuk add "mi'Kol ha'Beheimah Asher al ha'Aretz"?
Ramban #1: It simply means from all the animals in the world.
Why does the Torah insert the Parshah of Tahor and Tamei animals in Seifer Vayikra?
Ramban: Refer to 11:2:1:2.
Seforno: Refer to Yisro 20:21:5:4*. Because, following the episode of the Golden Calf, when Hashem withdrew His Shechinah from Yisrael, Moshe, with his Tefilos, succeeded in bringing it back in a limited manner - via the Mishkan - but only on condition that Yisrael live up to it by maintaining a high standard of spirituality, incorporating avoiding Tum'ah 1 and abstaining from sins that the Torah considers abominable. 2
Such as the Tamei species of animals and beasts, the specified eight species of vermin and the Neveilah of Tamei birds.
Refer to 11:11:1:2*. It also includes most of the Mitzvos contained in the current Parshiyos until and including Kedoshim, which lead to Kedushah (See Seforno in detail).
