Classes that became castes

First, there were four classes.' Particular attention has to be paid to the fact that this was essentially a class system, in which individuals, when qualified, could change their class, and therefore classes did change their personnel. At some time in the history of the Hindus, the priestly class socially detached itself from the rest of the body of people and through a closed-door policy became a caste by itself . The other classes being subject to the law of social division of labour underwent differentiation, some into large, others into very minute groups. The Vaishya and Shudra classes were the original inchoate plasm, which formed the sources of the numerous castes of today. As the military occupation does not very easily lend itself to very minute sub-division, the Kshatriya class could have differentiated into soldiers and administrators.
This sub-division of a society is quite natural. But the unnatural thing about these sub-divisions is that they have lost the open-door character of the class system and have become self-enclosed units called castes. The question is: were they compelled to close their doors and become endogamous, or did they close them of their own accord ? I submit that there is a double line of answer: Some closed the door : Others found it closed against them.'
An explanation of how classes turned into castes.