The Second edition of this Essay appeared in 1937, and was exhausted within a very short period. A new edition has been in demand for a long time. It was my intention to recast the essay so as to incorporate into it another essay of mine called " Castes in India, their Origin and their Mechanism ", which appeared in the issue of the Indian Antiquary Journal for May 1917. But as I could not find time, and as there is very little prospect of my being able to do so and as the demand for it from the public is very insistent, I am content to let this be a mere reprint of the Second edition.I am glad to find that this essay has become so popular, and I hope that it will serve the purpose for which it was intended.