In plain words
It is rare for purely sattvic people to exist in the world; among such people, mixtures of rajas and sattva exist too. Such people, though occupying a high station by birth, engage in rajasic actions by deed. The Veda shows, without concealing it, that these too are mixtures of varṇa. The Veda is impartial; it does not praise one person or disparage another -- that does not happen. It simply states the truth as it is. Here is a small example. Kauśika was a kṣatriya, that is, one full of rajo-guṇa. However, as the fruit of deeds across several births, sattvic feelings entered into him, and he moved about as one devoted to Satya. With a pure mind, he transformed his own life. The very mantra he uttered was the Gāyatrī Mantra; the author of this mantra is none other than Kauśika himself, and he himself is Viśvāmitra -- he became a friend to the whole universe. All brāhmaṇas receive that mantra as a sacred gift, practice it, and delight in it. This man was a kṣatriya by birth alone, but a brāhmaṇa by deed. This the Smṛti accepts. But the Veda, being the divine voice, preaches equal justice to all human beings; there is not the slightest partiality in it. Birth-castes arise following the jīva's own karma and guṇas.
Here, modern people of this age may commonly have a doubt -- and that is only natural. What is that doubt? If it is true that varṇa-divisions exist within creation itself, then, one might ask, shouldn't these exist in all countries too? Why should they exist only in Bhārata? Do all the things that exist in creation exist in all places too? They cannot. In accordance with their own region, their own climate, and their own laws, rules come to be formed suitably for their own places -- that is simply nature's own way. There is no rule that all the tree-species found in the Bhārata continent must also exist in other continents. It is not possible to wish that the stars in the sky should be on the earth, and for the fish that live in water