Are there any instances in Śruti scriptures which explicitly support birth-based varna?
Edit: I'm not looking for interpretations, please only cite those references which exactly say that Varna is birth-based i.e dependent upon lineage.
There's nothing in Shruti that supports the claim that Varna is based on birth. I cannot provide any citation as none exist. You might ask why didn't Shruti say anything about the birth based Varna. Well the reason is that Shruti is meant for reasonable human beings. It's sole purpose is to tell us who we are.
Prajnanam Brahma (प्रज्ञानम् ब्रह्म)
Aham Brahma Asmi (अहम् ब्रह्म अस्मि)
Tat Tvam Asi (तत् त्वम् असि)
Ayam Atma Brahma (अयम् आत्मा ब्रह्म)
It's not for those who see differences even on basis of birth.
Vasistha in Maharamayan says
Even human compositions are acceptable when they instruct good sense because men are always required to abide by reason. Otherwise, the Vedas should be renounced as unreliable. (Verse 2)
Words that conform to reason are to be received even if spoken by children. Otherwise they are to be rejected as straw even if they are pronounced by the lotus-born Brahma himself. (Verse 3)
(Chapter 18, Book 2)
Yes, the Taittiriya Samhita of the Krishna Yajur Veda states explicitly that castes are based on birth:
7.1.1 - Prajapati desired, 'May I have offspring.' He meted out the Trivrt from his mouth. After it the god Agni was created, the Gayatri metre, the Rathantara Saman, of men the Brahman, of cattle the goat; therefore are they the chief, for they were produced from the mouth. From the breast and arms he meted out the Pañcadaça Stoma. After it the god Indra was created, the Tristubh metre, the Brhat Saman, of men the Rajanya, of cattle the sheep. Therefore they are strong, for they were created from strength. From the middle he meted out the Saptadaça Stoma. After it the All-gods as deities were created, the Jagati metre, the Vairupa Saman, of men the Vaiçya [Vaishya], of cattle cows. Therefore are they to be eaten, for they were created from the receptacle of food. Therefore are they more numerous than others, for they were created after the most numerous of the gods. From his feet he meted out the Ekavinça Stoma. After it the Anustubh metre was created, the Vairaja Saman, of men the Çudra, of cattle the horse. Therefore the two, the horse and the Çudra [Shudra], are dependent on others. Therefore the Çudra is not fit for the sacrifice, for he was not created after any gods. Therefore they depend on their feet, for they were created from the feet.
When you rephrase the paragraph by keeping only the sentences related to caste, you end up with this:
Prajapati desired, 'May I have offspring.'
He meted out Brahmanas from his mouth. Therefore, they are the chief, for they were produced from the mouth.
From the breast and arms he meted out the Rajanya [or Kshatriyas]. Therefore, they are strong, for they were created from strength.
From the middle [waist or thighs] he meted out the Vaishyas. Therefore, they are more numerous than others, for they were created after the most numerous of the gods.
From his feet he meted out the Shudras. Therefore, Shudras are dependent on others. Therefore the Shudra is not fit for the sacrifice [Yajna], for he was not created after any gods. Therefore they depend on their feet, for they were created from the feet.
Since the castes were created or produced from the body of Prajapati just like offspring, they are based on birth; i.e. the sons and daughters of Brahmin parents are also Brahmins, and so on. Creation and production is a one-time event that is fixed. This implies that caste cannot change through ordinary means in life. There are exceptional situations like king Kaushitaki transforming into the Brahmana known as Vishvamitra, but that happened because Brahma give him a new Brahmana body from a Kshatriya body.
Another thing to consider is that the actual genetic makeup of the 4 castes are different. Brahmanas have Nadis in their body situated in such a way as to receive the vision of the Lord. Kshatriyas are genetically strong and physically fit. Vaishyas have a mind and body conducive for conducting business. Shudras are naturally pessimistic, lazy, simple-minded, and materialistic. That's how their bodies were constructed.
Manusmriti (10.5).- In all castes (varna) those (children) only which are begotten in the direct order on wedded wives, equal (in caste and married as) virgins, are to be considered as belonging to the same caste (as their fathers)
Āpastamba (2.13.1).—‘Sons begotten by a man who approaches in the proper season a woman of equal caste, who has not belonged to another man, and who has been married legally, have a right to follow the occupations of their castes.’
Viṣṇu (16.1).—‘On women equal in caste to their husbands, sons are begotten who are equal in caste to their fathers.’
Yājñavalkya (1.90).—‘From women of the same caste as their husbands are born sons of the same caste.’
Baudhāyana (1.17.2).—‘Sons of equal caste spring from women of equal caste.’
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