In the Kumbakonam lecture of 1897, Swami Vivekananda said the following:
We read in the Mahâbhârata that the whole world was in the beginning peopled with Brahmins, and that as they began to degenerate, they became divided into different castes, and that when the cycle turns round, they will all go back to that Brahminical origin. This cycle is turning round now, and I draw your attention to this fact. Therefore our solution of the caste question is not degrading those who are already high up, is not running amuck through food and drink, is not jumping out of our own limits in order to have more enjoyment, but it comes by every one of us, fulfilling the dictates of our Vedantic religion, by our attaining spirituality, and by our becoming the ideal Brahmin. There is a law laid on each one of you in this land by your ancestors, whether you are Aryans or non-Aryans, Rishis or Brahmins, or the very lowest outcasts. The command is the same to you all, that you must make progress without stopping, and that from the highest man to the lowest Pariah, every one in this country has to try and become the ideal Brahmin. This Vedantic idea is applicable not only here but over the whole world. Such is our ideal of caste as meant for raising all humanity slowly and gently towards the realisation of that great ideal of the spiritual man who is non-resisting, calm, steady, worshipful, pure, and meditative. In that ideal there is God.
Which Mahabharata verse does Swamiji refer to here?