SB 9.20.1
sri-badarayanir uvaca
puror vamsam pravaksyami
yatra jato ’si bharata
yatra rajarsayo vamsya
brahma-vamsyas ca jajñire
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sukadeva Gosvami said: O Maharaja Pariksit, descendant of Maharaja Bharata, I shall now describe the dynasty of Puru, in which you were born, in which many saintly kings appeared, and from which many dynasties of brahmanas began.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There are many historical instances by which we can understand that from ksatriyas many brahmanas have been born and that from brahmanas many ksatriyas have been born. The Lord Himself says in Bhagavad-gita (4.13), catur-varnyam maya srstam guna-karma-vibhagasah: “According to the three modes of material nature and the work ascribed to them, the four divisions of human society were created by Me.” Therefore, regardless of the family in which one takes birth, when one is qualified with the symptoms of a particular section, he is to be described accordingly. Yal-laksanam proktam. One’s place in the varna divisions of society is determined according to one’s symptoms or qualities. This is maintained everywhere in the sastra. Birth is a secondary consideration; the first consideration is one’s qualities and activities.