Naturally, if somebody is the son of a medical man his father trains him to become a medical man in future. That is the natural tendency. If the boy is different altogether, that is another question; but naturally, that is the tendency. So a kṣatriya's son was trained as kṣatriya, a brahmin's son was trained as a brahmin and a vaiśya's son was trained as a vaiśya, and śūdra had no training.
So gradually this became a caste system. brahmin's son became brahmin. Because formerly, the training was there. But when it is vitiated, although a person born in the family of a brahmin, he is doing the work of a śūdra. So according to Vedic scripture, one is classified according to his work and quality, not by birth. That is the classification of śāstra. Just like in the Bhagavad-gītā the Lord says, cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ (BG 4.13).
Guṇa means quality, and karma means work. One must be qualified for the work, and he must actually work. Then he is counted classified into that, I mean to say, category.
Just like if you are simply trained or educated as a lawyer, and if you are not practicing in the court, nobody comes to you to consult as a lawyer. Nobody cares for you. You must be practicing also. Similarly, to become a brahmin means first of all, he must know what is Brahman and he must be actually situated in the activities of Brahman.
So devotional service are activities of Brahman. Activities in Kṛṣṇa consciousness means activities in Brahman. Brāhme carati iti brahmā brahmacārī. Carati means acts. Actually he acts in life, applies the principles of brahmin in his life, he is called brahmacārī. So these were the trainings.
So just see how the training was, that a kṣatriya cannot refuse his promise. So Mahārāja Daśaratha, he fulfilled the promise of his youngest wife and asked his son, eldest son, Rāmacandra, "My dear boy, You'll have to go to forest for fourteen years. That is the desire of Your youngest mother. And I promised that I shall fulfill her promise, uh, request. So please accept." Rāmacandra said, "Yes, father, I am ready." Just see.