Mahabharata 3.203.11

शूद्रयोनौ हि जातस्य सद्गुणानुपतिष्ठतःवैश्यत्वं भवति ब्रह्मन्क्षत्रियत्वं तथैव च

śūdrayonau hi jātasya sadguṇānupatiṣṭhataḥvaiśyatvaṃ bhavati brahmankṣatriyatvaṃ tathaiva ca

Our plain-language meaning of this verse is waiting for scholar review.