Mbh 13.143.2a

"'"'The illustrious one said, "The status of a Brahmana, O goddess, is exceedingly difficult to attain. O auspicious lady, one becomes a Brahmana through original creation or birth. After the same manner the Kshatriya, the Vaisya, and the Sudra, all become so through original creation. Even this is my opinion[569]. He, however, that is born a Brahmana falls away from his status through his own evil acts. Hence, the Brahmana, after having attained to the status of the first order, should always protect it (by his acts). If one, who is a Kshatriya or Vaisya, lives in the practice of those duties that are assigned to the Brahmana, after the manner of a Brahmana he becomes (in his next life) a Brahmana. That Brahmana who casts off the duties of his order for following those assigned for the Kshatriya, is regarded as one that has fallen away from the status of a Brahmana and that has become a Kshatriya. That Brahmana of little understanding, who, impelled by cupidity and folly, follows the practices assigned to Vaisyas forgetful of his status as a Brahmana that is exceedingly difficult to attain, comes to be regarded as one that has become a Vaisya. Similarly, one that is a Vaisya by birth may, by following the practices of a Sudra, become a Sudra. Indeed, a Brahmana, falling away from the duties of his own order, may descend to the status of even a Sudra. Such a Brahmana, falling away from the order of his birth and turned out of it, without attaining to the region of Brahmana (which is his goal if he duly observes his own duties), sinks into Hell and in his next birth becomes born as a Sudra. A highly blessed Kshatriya or a Vaisya, that abandons those practices of his that are consistent with the duties laid down for his order, and follows the practices laid down for the Sudra, falls away from his own order and becomes a person of mixed caste. It is in this way that a Brahmana, or a Kshatriya, or a Vaisya, sinks into the status of a Sudra.