What is this manuvadi caste system? It is just about telling that son should do father's job or that king's son should be the king.
now see what Karunanidhi is doing? He is just following the Manu's law, he is crowning his son as CM, his other son as unofficial don of madurai and his daughter as the DMK's representative in Center.
DMK is thus following same Manu's laws it said it opposed. Can DMK make someone from ordinary background its CM? They cant, they wont?
