Does South India comes within land of mlechhas?
No, South India is not the land of mlechhas. Why? Because people possessing vedic knowledge reside in South India also. What is the pramANa for all this? It is Shankara's commentary on brihadaranyaka upanishad 1.3.10.
सा वा एषा देवतैतासां देवतानाम् पाप्मानम् मृत्युमपहत्य यत्रासां दिशामन्तस्तद्गमयांचकार, तदासां पामनो विन्यदधात्; तस्मान्न जनमियात्, नान्तमियात्, नेत्पाप्मानम् मृत्युमन्ववायानीति ॥ १० ॥
sā vā eṣā devataitāsāṃ devatānām pāpmānam mṛtyumapahatya yatrāsāṃ diśāmantastadgamayāṃcakāra, tadāsāṃ pāmano vinyadadhāt; tasmānna janamiyāt, nāntamiyāt, netpāpmānam mṛtyumanvavāyānīti || 10 ||
- This deity took away death, the evil of these gods, and carried it to where these quarters end. There it left their evils. Therefore one should not approach a person (of that region), nor go to that region beyond the border, lest one imbibe that evil, death.
....What did the vital force do after taking away death, the evil of the gods? It carried it to where these quarters, east and so forth, end. One may question how this was done, since the quarters have no end. The answer is that it is all right, for the quarters are here conceived as being that stretch of territory which is inhabited by people possessing Vedic knowledge; hence ‘the end of the quarters’ means the country inhabited by people who hold opposite views, as a forest is spoken of as the end of the country. Carrying them, there it, the deity, vital force, left their evils, the evils of these gods....
Blackbucks are native to the Indian subcontinent
Blackbuck is the state animal of Andhra Pradesh.
Therefore, Manusmriti is not saying that South India is land of mlechchas.
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