The fourth period in the development of the middle class extends from 1700 to the Revolution. It is marked by a split in the class, some of the small planters becoming wealthy, others failing to advance in prosperity, while still others degenerated, falling into abject poverty. This was almost entirely the result of the substitution of slave labor for the labor of the indentured servant. The importation of negroes had begun early in the 17th century, but for many years their numbers were so few that the vast bulk of the work in the fields had been performed by white men. In 1625 there were about 465 white servants in Virginia and only 22 negroes.[217] In 1649, when the population of the colony was 15,000, there were but 300 slaves.[218] In 1671, Governor Berkeley stated that there were only 2,000 slaves in Virginia, although the population was at that date about 40,000.[219] Near the end of the century, the number of negroes brought to the colony increased very much. The Royal African Company, which had obtained the exclusive right to trade in slaves with the English possessions, stimulated this human traffic to such an extent that negroes were soon found in every part of Virginia. By the year 1700 the number of slaves was about 6,000.[220] The negroes proved more suited to the needs of the planters than the white servants, for they served for life, were docile and easy to manage, stood well the unhealthful conditions in the tobacco fields, and, most important of all, they cheapened vastly the cost of production. The wealthy planters who had for so many years been limited in the amount of land they could place under cultivation by the number of servants they could procure, now found it possible to extend the scope of their operations. Before the end of the century such men as Byrd and Carter and Fitzhugh owned scores of slaves. It was this circumstance more than any thing else that accounts for the increased prosperity of the colony which is so noticeable during the first quarter of the 18th century.[221]