Nevertheless, the colonial legislators pressed relentlessly towards a two-tier system of black/white relations. In a two-tier system the mixed individual is considered black. This being the case and because the features and skin color of mixed individuals vary so greatly, the precise dividing line between the two races became a critical issue. For mixed individuals it meant the difference between freedom and enslavement. For whites it was crucial to maintaining the purity of the white race. By the end of the 17th century, colonial America’s system of indentured servitude had been completely replaced by chattel slavery, focusing almost exclusively upon blacks and mulattoes. Whites were no longer kept as servants. Furthermore, there was no longer any perceivable distinction between slave and servant. Black and mulatto slaves/servants alike were required to serve in perpetuity.