Mbh 13.143.1

"'"'Uma said, "O holy one, O thou that didst tear off the eyes of Bhaga and the teeth of Pushan, O destroyer of the sacrifice of Daksha, O three-eyed deity, I have a great doubt. In days of yore, the Self-born One created the four orders. Through the evil consequence of what acts doth a Vaisya become a Sudra? Through what acts doth a Kshatriya become a Vaisya and a regenerate person (Brahmana) becomes a Kshatriya? By what means may such degradation of castes be prevented? Through what acts does a Brahmana take birth in his next life, in the Sudra order? Through what acts, O puissant deity, does a Kshatriya also descend to the status of Sudra? O sinless one, O lord of all created beings, do thou, O illustrious one, dispel this doubt of mine. How, again, can the three other orders naturally succeed in attaining to the status of Brahmanhood?"