In most of the colonies import duties, and in at least one instance export duties, were levied on Indian slaves brought into or taken from the colonies. Such duties were generally levied for self-defense, though occasionally for revenue. During the colonial period England’s interest in the African slave trade led her to take effective measures to dispose of as many negroes as possible in the American colonies. In course of time the colonists awoke to the danger which might result from an excess of an ignorant servile class which in some sections outnumbered the white population. Frequent attempts were made in various colonies to check the importation of negroes by levying import duties. At times Indians as well as negroes were included in these laws. In their enactment it seems probable that the colonial legislatures had a double purpose: to shut out undesirables of both races, and to prevent the importation of negroes in the guise of Indians. Real danger threatened the colonies from an excessive importation of Indians as slaves, and an attempt was therefore made to check it. In those colonies where import duties furnished a substantial part of the colonial revenue, such duties were levied on Indian slaves as well as on other property. As early as 1698 the importation of negroes into South Carolina had reached such proportions that the safety of the province was considered endangered.[840] Attempts to check such importation were accordingly made throughout the colonial period by levying import duties. As the number of Indian slaves in the colony increased, they too were included as a basis for duties. By the act of 1721, the importation of negro, mulatto, mustee and Indian slaves (Spanish Indians excepted) by their owners was permitted without duty, provided such owner intended to settle in the colony and employ the slaves in his own service. He was required, however, to take an oath that in case he sold any of these slaves within twelve months after bringing them into the colony, he would pay certain required duties.[841]