In plain words
The cow and the horse are of the same animal kind. But by their guṇas themselves, one is fit for milk and the other is fit for riding. In this same way, human beings were divided not on the basis of their species, nor on the basis of their birth, but precisely on the basis of their guṇa and karma — these, Brāhmaṇa, Kṣatriya, Vaiśya and Śūdra and the rest. One cannot help saying that these are precisely divine determinations.