There are regulative principles. Just like our Society members, simply by being a member of the Society, being situated, is not all. He has to keep himself always alert in the principles. Then he is all right. Otherwise he is fallen down. Similarly, in the Śrīmad-Bhāgavatam and all Vedic scripture it is stated that the brahmin, kṣatriya, vaiśya, the higher class, how they can keep up their status. You must keep your status.
Just like in England the rule is that when a man is recognized as knight or lord by his activities, in order to continue the family as lord family, they have to deposit a certain amount of money so that they can continue to keep up the position of lord family. That amount should be deposited in the government treasury. So you cannot withdraw that, but you can enjoy the interest of it. It is a huge amount. Otherwise how he can keep up the status of lord family? And the eldest son of the family is awarded the title "lord," and he gets that interest and keeps up the status. Perhaps you know it.
So simply to take birth in a big family or a high family is not all. One has to qualify himself and keep up the status, the tradition, of the family. Then he is all right. Otherwise . . . similarly, the opposite side: a man may be born in a family of dog-eaters, but if he keeps himself as dog-eater, then he is dog-eater family.
But if he changes his status—no more dog-eater, but he is a devotee— he is not counted as belonging to that family, either this side or that side. A man born in a high family, if he is unable to keep up the status of the standard of living and tradition of that high family, then he does not belong.
Similarly, a man born in the lower family, if he does not keep to the status of that lower family—he takes to the standard of brahmin family, Vaiṣṇava family—then he is to be considered belonging to that status. Not that because he is born in that dog-eater's family he cannot become a brahmin. No. This is not the Vedic injunction.
A person born in the dog-eater's family can be raised to the standard of brahmin family, or a person born in the brahmin family can be lowered down to the status of dog-eater's family if he does not keep the status. So it is the qualification. That is Vedic.