तस्मिन् देशे य आचारः पारम्पर्यक्रमागतः । वर्णानां सान्तरालानां स सदाचार उच्यते ॥ १८ ॥
The custom handed down in regular succession (since time immemorial) among the (four chief) castes (varna) and the mixed (races) of that country, is called the conduct of virtuous men.