In the Bhagavad Gita, Ch.4, Verse 13
The Lord says:“The fourfold caste has been created by Me according to the differentiation of Guna and Karma;”

In the Bhagavad Gita, Ch.18, verse 40
the Lord says:
“There is no being on earth, or again in heaven among the gods, that is liberated from the three qualities born of Nature.”

Gita Ch. 18, V.41:
“Of Brahmanas, Kshtriyas and Vaishyas, as also the Sudras, O Arjuna, the duties are distributed according to the qualities born of their own nature.”

Note: The word ‘GUNA’  in Vedanta means prakriti (Satwa, Rajas and Tamas). The principle of caste system is not confined to any one country. It applies to all human beings of all races, in all countries. This Sanatan principle is eternal and is not subject to change. It remains unchanged in the past, in the present and in the future.

The following explanation is from The Mahabharata,

The Four Orders of human Beings
From The Mahabharata
Santi Parva, Section CLXXXVIII
Translated by Sri Kisari Mohan Ganguli

Brigu said, “….. (The Creator created) human beings with their four divisions, viz., Brahmanas, Kshatriyas, Vaisyas, and Sudras. The complexion the Brahmanas obtained was white; that which the Kshatriyas obtained was red; that which the Vaisyas got was yellow; and that which was given to the Sudras was black.”

Bhrigu said, “That person is called a Brahman who has been sanctified by such rites as those called JATA and others; who is pure in behaviour; who is engaged in studying the Vedas; who is devoted to the six well-known acts (of ablutions every morning and evening, silent recitation of mantras, pouring libations on the sacrificial fire, worshipping the deities, doing the duties of hospitality to guests, and offering food to the Viswedevas); who is properly observant of all pious acts; who never takes food without having offered it duly to gods and  guests; who is filled with reverence for his preceptor; and who is always devoted to vows and truth.

He is called a Brahmana in whom are truth, gifts, abstention from injury to others, compassion, shame, benevolence and penance.

He who is engaged in the profession of battle, who studies the Vedas, who makes gifts (to Brahmanas) and takes wealth (from those he protects) is called a Kshatriya.

He who earns fame from keep of cattle, who is employed in agriculture and the means of acquiring wealth, who is pure in behaviour and attends to the study of the Vedas, is called a Vaisya.

He who takes pleasure in eating every kind of food, who is engaged in doing every kind of work, who is impure in behaviour, who does not study the Vedas, and whose conduct is unclean, is said to be a Sudra.

If these characteristics be observable in a Sudra, and if they be not found in a Brahmana, then such a Sudra, is no Sudra, and such a Brahmana is no Brahmana.  By every means should cupidity and wrath be restrained.

This as also self-restraint, are the highest results of Knowledge. Those passions (cupidity and wrath), should, with one`s whole heart, be resisted. They make their appearance for destroying one`s highest good.

One should always protect one`s prosperity from one`s wrath, one`s penance from pride; one`s knowledge from honour and disgrace; and one`s soul from error.  That intelligent person, who does all acts without desire of fruit, whose whole wealth exists for charity, and who performs the daily Homa, is a real renouncer (karma-sannyasa).

One should conduct oneself as a friend to all creatures, abstaining from all acts of injury. Rejecting the acceptance of all gifts, one should, by the aid of one`s intelligence, be a complete master of one`s passions. One should live in one`s soul where there can be no grief. One would then have no fear here and attain to a fearless region hereafter. One should live always devoted to penances, and with all passions completely restrained; observing the vow of taciturnity, and with soul concentrated on itself; desirous of conquering the unconquered senses, and unattached in the midst of attachments. The indications of a Brahmana are purity, good behaviour and compassion unto all creatures.”

Sufficient liberty of action is left to them in consequence of which all individuals may attain to an equality of condition.

From The Mahabharata
Anusasana Parva, section CLXIV
Translated by Sri Kisari Mohan Ganguli

Bhishma said:  A man of wisdom cannot catch hold of a sinful person and forcibly cause him to become righteous. When seriously urged to act righteously, the sinful only act with hypocrisy, impelled by fear. They that are righteous among the Sudras never betake themselves to such hypocrisy under the plea that persons of the Sudra order are not permitted to live according to any of the four prescribed modes. I shall tell thee particularly what the duties truly are of the four orders.

So far as their bodies are concerned, the individuals belonging to all the four orders have the five primal elements for the constituent ingredients. Indeed, in this respect, they are all of the same substance. For all that, distinctions exist between them in respect of both practices relating to life or the world and the duties of righteousness. Notwithstanding these distinctions, sufficient liberty of action is left to them in consequence of which all individuals may attain to an equality of condition. 

All men are equal in respect of their physical organism.All of them, again, are possessed of souls that are equal in respect of their nature.

When dissolution comes, all else dissolve away. What remains is the inceptive will to achieve Righteousness. That, indeed, reappears (in next life) of itself. When such is the result (that is when the enjoyments and endurance of this life are due to the acts of a past life), the inequality of lot discernible among human beings cannot be regarded in any way anomalous.

The following explanation is from The Mahabharata:

The status of a Brahmana is incapable of acquisition by persons begotten on uncleansed souls.

Kshatriya, Vaishya and Sudra cannot become Brahman

From The Mahabharata
Anusasana Parva, sec.XXVIII,
Translated by Sri Kisari Mohan Ganguli  

Bhishma said: The status of a Brahmana is incapable of acquisition by a person belonging to any of the three other orders. That status is the highest with respect to all creatures. Travelling through innumerable orders of existence, by undergoing repeated births, one at last, in some birth, becomes born as a Brahmana. The status of a Brahmana is incapable of acquisition by persons begotten on uncleansed souls. The status of a Brahmana, which is the foremost of everything, is incapable of being won by penances. That status is really incapable of being obtained by persons of uncleansed souls.

But with the aid of these acts a Sudra may become a Brahman

Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status.

From the Mahabharata
Anusasana Parva, Section CXLIII
Translated by Sri Kisari Mohan Ganguli

Addressing Uma, Mahadeva said: Endued with knowledge and science, purified from all dross, and fully conversant with the Vedas, a pious Kshatriya, by his own acts, becomes Brahman.

If the Sudra desires to be a Vaisya, he should abstain from meat of animals not slain in sacrifices. He should be truthful in speech, and free from pride and arrogance. He should rise superior to all pairs of opposites (such as heat and cold, joy and sorrow etc.), he should be observant of the duties of peace and tranquillity. He should adore the deities in sacrifices, attend with devotion to the study and recitation of the Vedas, and become pure in body and mind. He should keep his senses under subjection, honour the Brahmanas, and seek the welfare of all the orders.

Leading the domestic mode of life and eating only twice a day at the prescribed hours he should gratify his hunger with only such food as remains after the needs have been satisfied of all the members of his family with dependants and guests. He should be abstemious in food, and act without being impelled by the desire of reward. He should be free from egotism. He should adore the deities in Agnihotra (Sacred Fire Ceremony) and pour libations according to the ordinance. Observing the duties of hospitality towards all persons, he should, as already said, eat the food that remains after serving all others for whom it has been cooked. He should, according to the ordinance laid down, worship the three fires.

If a Vaisya,(after becoming a Kshatriya, wishing to become a Brahmana), goes through the usual purificatory rites, becomes invested with the sacred thread, and betakes himself to the observance of vows, he should make presents, adore the deities in great sacrifices with plentiful Dakshinas (gifts), study the Vedas. He should interfere for dispelling the sorrows of the distressed and should always righteously cherish and protect those subjects that own his sway. He should be truthful and do all acts that have truth in them and seek happiness in conduct like this. He should induce men to do righteous deeds.

He should never indulge in sexual pleasure, but live cheerfully and in independence, well conversant with the science of Wealth and Profit. Of righteous soul, he should seek his wedded spouse only in her season. He should always observe fasts, keep his soul under control, devote himself to the study of the Vedas, and be pure in body and mind.

He should pursue the aggregate of Three (Virtue, Wealth and Pleasure), and be always cheerful. He should never desire anything from motives of gain or pleasure. He should worship the Pitris (ancestors) and gods and guests.

In his own house, he should live the life of a mendicant. He should duly adore the deities in his Agnihotra, morning, noon and evening every day, by pouring libations agreeably to the ordinance.

Endued with knowledge and science, purified from all dross, and fully conversant with the Vedas, a pious Kshatriya, by his own acts, becomes a Brahmana. It is with the aid of these acts, that a person who has sprung from a degraded order, viz., a Sudra, may become a Brahmana refined of all stains and possessed of Vedic lore.

One that is Brahmana, when he becomes wicked in conduct and observes no distinction in respect of food, falls away from the status of a Brahmanhood and becomes a Sudra.

Even a Sudra that has purified his soul by pure deeds and that has subjugated all his senses, deserves to be waited upon and served with reverence as a Brahmana.. This has been said by the Self-born Brahman Himself. When a pious nature and pious deeds are noticeable in even a Sudra, he should be held superior to a person of the three regenerate classes. Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status. Verily conduct is the only ground.

All Brahmanas in this world are Brahmanas in consequence of conduct. A Sudra, if he is established on good conduct, is regarded as possessed of the status of a Brahmana. The status of a Brahmana is equal wherever it exists. He, indeed, is a Brahmana in whom the status of Brahma exists; that condition which is bereft of attributes and which has no stain attached to it.

The boon giving Brahma, while he created all creatures, himself said that the distribution of human beings into the four orders dependent on birth is only for purposes of classification. The Brahmana who wishes to achieve his own good should always adhere to the path of righteousness. He should always be devoted to the study of the Vedas, but he should never derive the means of sustenance from such study.

That Brahmana who always conducts himself thus, adhering to the path of righteousness, worshipping his sacred fire, and engaged in the study of the Vedas, comes to be regarded as Brahma.

The status of Brahmana once gained, it should always be protected with care by avoiding the stain of contact with persons born in inferior orders, and by abstaining from the acceptance of gifts. I have thus told thee a mystery, viz., the manner in which a Sudra may become a Brahmana, or that by which a Brahmana falls away from his own pure status and becomes a Sudra.

The following explanation is from The Mahabharata,

From Tejabind upanishad, Shloka 12:

Brahman (the Supreme Reality) is not known to those who are possessed of avarice, delusion, fear, egotism, lust, anger, and sin or possessed of (unable to bear) heat and cold, hunger and thirst, or mental  resolve and indecision, or pride of birth in a Brahmin (priest) family, or vanity in having read a mass of books on Mukti (liberation or salvation).

Note: Comments by Swami Madhavananda, Advaita Ashrama: Realisation is not dependent on birth or book-learning as has been repeatedly demonstrated in the lives of saints, from the very earliest times to our own day.

From The Mahabharata
Santi Parva, Section CCXL:
Translated by Sri Kisari Mohan Ganguli

Vyasa said: Even if a person happens to belong to the inferior order (Sudra), even if one happens to be a woman, both of them, by following in the path of Yoga, will surely attain to the highest end.

Note: What is the path of Yoga? Vyasa  first provides the definition of the word ‘YOGA’ as follows: The uniting together of Intellect and Mind, and all the senses, and the all-pervading Soul is said to be Knowledge of the foremost kind.

Vyasa further explains the path of Yoga: That Knowledge should be acquired (through the preceptor’s aid) by one that is of a tranquil disposition, that has mastered his senses, that is capable (by meditation) of turning his gaze on the Soul, that takes a pleasure in such meditation, that is endued with intelligence and pure in acts.
One should seek to acquire this Knowledge by abandoning those five impediments of Yoga which are known to the wise, viz., desire, wrath, cupidity, fear and sleep.

By Swami Nikhilananda
Ramakrishna – Vivekananda Centre, New York
Extract from an essay on ‘Hindu Ethics’

The Basis of the Caste System

The basis of the caste system, according to the Hindu view, is men’s self-evident inborn inequality; physical, intellectual, and spiritual. An individual is born into a higher or lower caste as a result of actions performed by him in his previous life, and each person, therefore, is himself responsible for his position. By discharging the duties determined by his caste, a man becomes qualified for birth in a higher caste in a future life. If one does not accept the doctrine of rebirth and the law of karma, then the inequity from which members of lower castes often suffer cannot be explained.

Brahman must never accept food from a Sudra

Anusasana Parva, Section CXXXV
Translated by Sri Kisari Mohan Ganguli

Bhishma said: A brahmana may take his food from another Brahmana or from a Kshatriya or a Vaisya, but he must never accept food from a Sudra. A Kshatriya may take his food from a Brahmana, a Kshatriya or a Vaisya. He must, however, eschew food given by Sudras who are addicted to evil ways and who partake of all manner of food without any scruple.

Brahmanas and Kshatriyas can partake of food given by such Vaisyas as tend the sacred fire every day, as are faultless in character, and as perform the vow of Chaturmasya (Note: Chaturmasya is the four-month period of special vows and austerities in Hindu tradition). … Verily, those Brahmanas that take their food from Sudras, take the dirt of the earth. …. Those who impudently partake of food offered at ceremonials in a Sudra’s house are afflicted with a terrible calamity. In consequence of partaking  such forbidden food they lose their family, strength, and energy, and attain to the status of animals, descending to the position of dogs, fallen in virtue and devoid of all religious observances.

The acceptance of food from a wicked person is considered as reprehensible as the slaying of a Brahmana. One should not accept food if one is slighted and not received with due honours by the giver. A Brahmana, who does so, is soon overtaken by disease, and his race soon becomes extinct.

From The Mahabharata
Anusasana Parva, Section CXLIII,
Translated by Sri Kisari Mohan Ganguli

Addressing Uma, Mahadev said: The culture of the soul those that are righteous and desirous of acquiring merit always pursue with firmness the culture of the soul. The food that comes from cruel and fierce persons is censurable. So also is the food that has been cooked for serving a large number of persons. The same is said of the food that is cooked in view of the first Sraddha of a diseased person. So also is the food that is stained in consequence of the usual faults and the food that is supplied by a Sudra. These should never be taken by a Brahmana at any time.

The food of a Sudra, O goddess, is always disapproved of by the high-souled deities.  ….. In consequence of those remains of a Sudra’s food in his stomach, he falls away from the status of a Brahmana.

Such a Brahmana becomes invested with the status of a Sudra, there is no doubt in this. This Brahmana in his next life becomes invested with the status of that order upon whose food he subsists through life or with the undigested portion of whose food in his stomach he breathes his last.

That man who, having attained to the auspicious status of a Brahmana which is so difficult to acquire, disregards it and eats interdicted food, falls away from his high status. That Brahmana who drinks alcohol, who becomes guilty of Brahmanicide or mean in his behaviour, or a thief, or who breaks his vows, or becomes impure, or unmindful of his Vedic studies, or sinful, or characterised by cupidity, or guilty of cunning or cheating, or who does not observe vows, or who weds a Sudra woman, or who derives his subsistence by pandering to the lusts of other people, or who sells the Soma plant, or who serves a person of an order below his, falls away from his status of Brahmanhood.

That Brahmana who violates the bed of his preceptor, or who cherishes malice towards him, or who takes pleasure in speaking ill of him. falls away from the status of Brahmanhood even if he be conversant with Brahmana.  By these good acts, again, O goddess, when performed, a Sudra becomes a Brahmana. 

From The Mahabharata
Santi Parva, SectionXLVIII
Translated by Sri Kisari Mohan Ganguli

Bhishma said: 

O Krishna (O Supreme Lord), Brahmanas are thy mouth,
Kshatriyas are thy two arms, Vaisyas are thy stomach and
thighs, and Sudras live in thy feet.

From the Bhagavad Gita

In the Bhagavad Gita, Ch.18, verse 40
The Lord says:

“There is no being on earth, or again in heaven among the gods, that is liberated from the three qualities born of Nature.”

Gita Ch. 18, V.41:
“Of Brahmanas, Kshtriyas and Vaishyas, as also the Sudras, O Arjuna, the duties are distributed according to
the qualities born of their own nature.”

Gita Ch.18, v.42:
“Serenity, self-restraint, austerity, purity, forgiveness and also uprightness, knowledge, realisation and belief in God are the duties of the Brahmanas, born of their own nature.”

Gita,Ch. 18, v.43:
“Prowess, splendour, firmness, dexterity and also not fleeing from battle, generosity and lordliness are the duties of Kshatriyas, born of their own nature.”

Gita,Ch. 18, v. 44:
“Agriculture, cattle-rearing and trade are the duties of the Vaishya merchant class), born of their own nature; and action consisting of service is the duty of the  Sudras (servant class), born of their own nature.”

Gita, Ch.18, v.45:
“Each man devoted to his own duty, attains perfection.”

Gita, Ch. 18, v.47.:
“Better is one`s own duty (though) destitute of merits, than the duty of another well performed. He who does the duty ordained by his own nature incurs no sin.”

Gita, Ch. 18, v.48.:
“One should not abandon , O Arjuna, the duty to which one is born, though faulty; for, all undertakings are enveloped by evil, as fire by smoke.”

Swami Sivananda (The Divine Life Society, Rishikesh) , in his commentary on Gita,Ch.18, verses 41,and 45 says:

“Mankind is organised into the four castes and each man`s life is divided into four stages, according to the nature of the Gunas and the degree of growth or evolution. This is the division of labour for which each caste is fitted according to its own nature. The duty prescribed is your sole support, each devoted to his own duty in accordance with his own nature or caste, and the highest service you can render to the Supreme is to carry it out whole-heartedly, without expectation of fruits, with the attitude of dedication to the Lord.

The caste system is, indeed, a splendid thing. It is quite flawless. But the defect came in from somewhere else. The classes gradually neglected their duties. The test of ability and character slowly vanished. Birth became the chief consideration in determining castes. All castes fell from their ideals and forgot all about their duties.”

Mundaka Upanishad III-(1)-5

Truth, penance, understanding and purity are essential requisites for this revelation of the Brahman (the Supreme Reality) within. When the heart is cleansed, Brahman is revealed, and He is seen shining like a burning light within oneself.

Purity of Birth

From The Mahabharata
Anusasana Parva Section  XLIX.
Translated by Sri Kisari Mohan Ganguli

Bhishma said: A person that is born of irregular union presents diverse features of disposition. One`s purity of birth, again, is to be ascertained from one`s acts which must resemble the acts of those who are admittedly good and righteous. A disrespectable behaviour, acts opposed to those laid down in the scriptures, crookedness and cruelty, and abstention from sacrifices and other spiritual acts that lead to merit, proclaim one`s impurity of origin.

A son receives the disposition of either the father or the mother. Sometimes he catches the disposition of both.A person of impure birth can never succeed in concealing his true disposition. As the cub of a tiger or a leopard resembles its sire and dam in form and in (the matter of) its stripes or spots, even so  a person cannot but betray the circumstance of his origin. However covered may the course of one`s descent be, if that descent happens to be impure, its character or disposition is sure to manifest itself slightly or largely. A person may, for purposes of his own, choose to tread on an insincere path, displaying such conduct as seems to be righteous. His own disposition, however, in the matter of those acts that he does, always proclaims whether he belongs to a good order or to a different one.

Creatures in the world are endued with diverse kinds of disposition. They are again, seen to be employed in diverse kinds of acts. Amongst creatures thus employed, there is nothing that is so good or precious as pure birth and righteous conduct. If a person be born in a low order, that good understanding which arises from a study of the scriptures fails to rescue his body from low acts. Absolute goodness of understanding may be of different degrees. It may be high, middling or low. Even if it appears in a person of low extraction, it disappears like autumnal clouds without producing any consequences. On the other hand, that other goodness of understanding which, according to its measure, has ordained the status in which a person has been born, shows itself in his acts.

If a person happens to belong to a superior order but still if he happens to be divested of good behaviour, he should receive no respect or worship. One may worship even a Sudra if he happens to be conversant with duties and be of good conduct. A person proclaims himself by his own good acts and by his good or bad disposition and birth. If one`s race of birth happens to be degraded for any reason, one soon raises it and makes it resplendent and famous by one`s acts.
For these reasons they that are endued with wisdom should avoid those women, among these diverse castes mixed or pure, upon whom they should not beget offspring.

From The Mahabharata
Santi Parva, Section CCXCVII
Translated by Sri Kisari Mohan Ganguli

Janaka said: O great ascetic, is man stained by his acts or is he stained by the order or class in which he is born? A doubt has arisen in my mind. It behoveth thee to expound this to me.

Parsara said: Without doubt, O king, both, viz., acts and birth, are sources of demerit. Listen now to their difference. That man who, though stained by birth, does not commit sin, abstains from sin notwithstanding birth and acts. If, however, a person of superior birth perpetrates censurable acts, such acts stain him. Hence, of the two, viz., acts and birth, acts stain man more than birth.

Low Birth & High Birth

From The Mahabharata
Santi Parva, Section CXVIII
Translated by Sri Kisari Mohan Ganguli

Bhishma said: If a king gathers round him persons of low birth, he can never be happy. A person of high birth, even if persecuted without any fault by his royal master, never sets his heart, in consequence of the respectability of his blood, upon injuring his master. An individual, however, that is mean and of low birth, having obtained even great affluence from his connection with some honest man, becomes an enemy of the latter if only he is reproached in words.

An intelligent king should, guided by precedent, appoint servants, each fit for the office assigned to him, and exercise proper supervision over them. Having first ascertained their qualifications in respect of truthfulness and purity, general disposition, knowledge of the scriptures, conduct, birth, self-restraint, compassion, strength, energy, dignity, and forgiveness. A king should never take a minister without first having examined him. A minister should be possessed of high birth and strength.

Time Assails the Energy of Righteousness
Mahabharata, Anusasana Parva, Section XCIV
Translated by Sri Kisari Mohan Ganguli

Bhishma said: It is heard that Time assails the energy of Righteousness. That Time has come upon us. Hence, righteousness is afflicted. I should go to heaven for good, before unrighteousness assails the world and establishes itself here.

Note: The scriptures declare that Righteousness loses its strength as Time advances. In the Krita age, it exists in its entirety. In the Treta, it loses a quarter. In the Dwapar, another quarter is lost. In the Kali age, full three quarters are lost and only a quarter is all that remains.

Before the time comes when Brahmans, loudly uttering the Vedas, within the precincts of villages and inhabited places, cause the Sudras to hear them. Before men cease to regard the distinctions between the lower, the middle, and the higher classes,* I shall go to heaven for good. Before ignorance assails the world and envelopes all things in Darkness, before the time comes when the strong begin to lord it over the weak and treat them as slaves.

*(The scholar and translator of the Mahabharata, Sri Kisri Mohan Ganguli comments: `The Rishis think that  the distinctions between the lower, the middling, and the higher  classes of society are eternal, and nothing can be a greater calamity than the effacement of those distinctions. Equality of  men, in their eyes, is an unmitigated evil.’)

From Brahma-Sutras
Translation and explanations
by Swami Vireswarananda
Advaita Ashrama, Mayavati, Himalayas

Sudras are not entitled to the study of the Vedas

Brahma-Sutras  1-3-36:
Because purificatory ceremonies are mentioned (in the case of the twice born) and their absence are declared (in the case of the Sudras).
[Note: Purificatory ceremonies like Upanayana (Sacred Thread) etc. are declared by the scriptures to be a necessary condition of the study of all kinds of knowledge or Vidya; but these are meant only for the higher castes (Brahmins, Kshatriyas and Vaisyas). Their absence in the case of the Sudras is repeatedly declared in the scriptures. “Sudras do not incur sin (by eating prohibited food), nor have they any purificatory rights ” etc. (Manusmrti  10-12-6). Consequently they are not entitled to the study of the Vedas.]

Braham-Sutras 1-3-37
And because the inclination (on the part of Gautama to impart Knowledge is seen only) on the ascertainment of the absence of Sudra-hood (in Jabala Satyakama from the Chandogya Upanishad of the Sama Veda, 4-49.)

[Note: That Sudras are not qualified is known also from the fact that great teachers like Gautama made sure before imparting Knowledge that disciples like Jabala Satyakama were not Sudras.]

Brahma-Sutras 1-3-38
And because of the prohibition in the Smrti of hearing and studying (the Vedas) and knowing their meaning and performing their Vedic rites to Sudras, they are not entitled to the Knowledge of Brahman.
[Note: Sutras 34-38 of Brahma-Sutras disqualify the Sudras for the Knowledge of Brahman (Supreme Reality) through the study of the Vedas. But it is possible for them to attain that Knowledge through the Puranas and the epics (Ramayana and the Mahabharata).

From Sri Ramacharitamanasa of Tulasidas
Uttar-kanda, verses  97-98

In the Kali Yuga
No one follows the duties of one’s own caste, and the
four Ashrams or stages of life also disappear.

Sudras instruct the twice-born (Brahman, Kshatriya and Vaishya)
in spiritual wisdom and, wearing the sacred thread, accept the worst
type of gifts. (Verse  98)

From The Mahabharata
Santi Parva, Section CLXV, 
Translated by Sri Kisari Mohan Ganguli

Bhishma said:

The Sudra has no competence for performing a sacrifice.

From The Mahabharata
Santi Parva, Section CCXXXVIII
Translated by Sri Kisari Mohan Ganguli

Vyasa said:

In the Kali Yuga, the duties of the respective order
disappear and men become afflicted by inequity.

The Mahabharata
Udyoga Parva, Section XXIX
Translated by Sri Kisari Mohan Ganguli

Krishna Says:

The following are the duties declared for a SUDRA from the olden times.
He should serve the Brahmanas and submit to them; should not study; sacrifices are forbidden to him; he should be diligent and be constantly enterprising in doing all that is for his good. The king protects all these with proper care and sets all the castes to perform their respective duties.

A VAISYA should study and diligently earn and accumulate wealth by means of commerce, agriculture, and the tending of cattle. He should so act as to please the Brahmanas and the Kshatriyas, be virtuous, do good works, and be a householder.

A KSHATRIYA should protect the people in accordance with the injunctions of the law, diligently practise the virtue of charity, offer sacrifices, study the whole Veda, take a wife, and lead a virtuous householder’s life. If he be possessed of a virtuous soul, and if he practises the holy virtues, he may easily attain the religion of the Supreme Being.

A BRAHMAN should study (the Vedas), offer sacrifices, make charities, and sojourn to the best of all holy places on the earth; he should teach, minister as a priest in sacrifices offered by others worthy of such help, and accept gifts from persons who are known.

From The Mahabharata
Santi Parva, Section CCXCV
Translated by Sri Kisari Mohan Ganguli

Parsara said: If the Brahmana, pressed for a living, betakes himself to the duties of either the Kshatriya or the Vaisya, he does not fall off from righteousness. When, however, the Brahmana betakes himself to the duties of the lowest order, then does he certainly fall off. When the Sudra is unable to obtain his living by service of the three other orders, then trade, rearing of cattle, and the practice of the mechanical arts are lawful for him to follow.

Instructions Forbidden to These
The Mahabharata
Anusasana Parva, Section .X
Translated by Sri Kisari Mohan Ganguli

Addressing king Yudhishthira, Bhishma said:
“I shall recite to thee what I heard certain Rishis say in days of yore.
Instruction* should not be imparted unto one that belongs to a low or mean caste. It is said that the preceptor who imparts instruction to such a person incurs great fault.”

*(Note: A tumbler ( a glass for drinking water ); if this tumbler is full of dirt, grease and other  impurities, then it is an unfit receptacle for holding pure water. The mind is the container and if it is filled with Tamasic qualities, then it is an unfit receptacle for receiving pure spiritual knowledge.)

Pledge further that you would protect
the world from an intermixture of castes
The Mahabharata
Santi Parva, Section LIX
Translated by Sri Kisari Mohan Ganguli

Bhishma said: The son of Vena then, with joined hands, said unto those great Rishis: “I have attained an understanding that is very keen and that is observant of righteousness. Tell in detail what I shall do with it.“

Thus addressed, the gods that were present there, as also the Rishis, said unto him: “Do fearlessly accomplish all those tasks in which righteousness even resides. Disregarding what is dear and what not so, look upon all creatures with an equal eye. Cast off at a distance lust and wrath and covetousness and honour, and, always observing the dictates of righteousness, punish with your own hands the man, whoever he may be, that deviates from the path of duty. Also swear that you would, in thought, word and deed, always maintain the religion inculcated on earth by the Vedas. Do further swear that you would fearlessly maintain the duties laid down in the Vedas with the aid of the science of chastisement, and that you would never act with caprice. Know that Brahmanas are exempt from chastisement, and pledge further that you would protect the world from an intermixture of castes.”

From The Bhagavad Gita
Chapter 1, Verses 40 – 44

Addressing Lord Krishna

Arjuna said:

40. In the destruction of a family, the immemorial religious
    rites of that family perish; on the destruction of spirituality,
    impiety overcomes the whole family.
41. By the prevalence of impiety, O Krishna, the women of the
    family become corrupt; and women being corrupted, O Vaarshneya (descendant of Vrishni) there arises intermingling of castes.
42. Confusion of castes leads to hell the slayers of the family, for their forefathers fall, deprived of the offerings of rice-ball and water (libations).
43. By these evil deeds of the destroyers of the family, which cause confusion of castes, the eternal religious rites of the caste and the family are destroyed.
44. We have heard, O Janardana, that inevitable is the dwelling
    for an unknown period in hell for those men in whose families the religious practices have been destroyed.

Brahmanas and Sudras
From Tulasi Ramayana
Aranya Kanda 32-33

Addressing a Gandharva (Kabandha resurrected as Gandharva)

Sri Rama said: Listen, O Gandharva, to what I tell you: I cannot tolerate an enemy of the Brahamanas (Brahmins). He who without guile in thought, word and deed does service to the Brahmanas (the very gods on earth), wins over Brahma, Siva, Myself and all other divinities.

A Brahmana, even though he curses you, beats you or speaks harsh words to you, is still worthy of adoration: so declare the saints. A Brahmana must be respected, though lacking in amiability and virtue; not so a Sudra, though possessing a host of virtues and rich in knowledge.

The Four Varnas
Brahadaranyaka Upanishad Paraphrased- simplified- abridged

By R.R.Diwakar
Reproduced from our page Stories & Episodes (30)

[The fourfold division of Hindu society into Brahmin, Kshatriya, Vaisya and Sudra seems to be quite ancient. The Rg Veda mentions the division and says that these emerged from the different parts of the body of the Virata Purusha or the primeval mighty being. It is clear that originally the division was functional and not hereditary. Here is an explanation of that system given in an allegorical manner. It is said here that society is complete and perfect on account of the existence of all these four divisions but much more so on account of the law which binds all and which all ought to obey.]

The Creator Prajapati first created the god Brahma. He represented Intelligence. But the Creator was not satisfied with that only. He felt that he should create other gods also if creation were to be a complete manifestation of the various powers in him.

He created the Kshatriya gods, Indra, Varuna, Soma, Rudra, Parjanya, Yama and others. They were the embodiments of power, valour, brilliance, fearlessness, the tendency to govern, and such other qualities.

But the Creator was not satisfied even with these new gods. He saw that there was still something wanting in creation. So he created the Vaisya gods, the eight Vasus, Aditya, the Maruts and so on.

But then he saw that the society of gods he wanted to evolve was not yet complete. So he added Pusan to the creation. He represents the Sudra principle, namely, manual labour and service.

Even this did not satisfy the Creator. He therefore created Dharma or the Law that binds all, that keeps all in their own places and strengthens all who act according to it. Those who do not follow the Law fall away, however strong they might be. Those who follow the law are stronger than the strongest because they adhere to the law. He who speaks the law speaks the truth. He, who speaks the truth, speaks the law. Truth and the Law are one.

Corresponding to this creation of his in the heavens, Prajapati created human society also on the same pattern and laid down the law for all the four Varnas. The law lays down the functions of the four pillars of the social system. Those who follow the law and perform their functions accordingly have nothing to fear. They are stronger than the strongest and they are bound to be happier than the happiest.

Intelligence, sacrifice, disinterested service are the characteristics of the Brahmins. Valour, chivalry, forgiveness, ability to rule are the characteristics of the Kshatriyas. Trade, co-operation, agriculture and distribution of material wealth are the characteristics of the Vaisyas. Ungrudging manual labour and service are the characteristics of the Sudras.

To choose our functions according to our powers and to attune our powers to the functions that we take up, is the only way to follow the Law and maintain social harmony.