The Vedic people stopped interbreeding with the earlier local population and began to talk of purity only when they no longer needed women from outside their community as wives, because they now had enough girl-children whose early mixed roots, they decided, did not matter. And the British came up with their racist notions of not mixing with Indians only after the Suez Canal opened and there were fast steamships bringing white British women to India in search of British husbands.
