These conclusions are borne out by the slowness with which the population increased during the years following the passage of the Navigation Acts. In the Commonwealth period the colony had advanced by leaps and bounds, and the inhabitants, estimated at 15,000 in 1649,[5-38] were placed by Berkeley thirteen years later at 40,000.[5-39] Under the system which existed during these years, when the colonists enjoyed a comparatively free trade, the population had tripled. But after 1660, while the Virginia tobacco was dumped upon the restricted English market and prices fell lower and lower, no such rapid growth is noted. In 1671, nine years after his first estimate, Governor Berkeley still placed the population at 40,000.[5-40] And even if we accept the statement of the Virginia agents sent to England to secure a charter for the colony that in 1675 the number of inhabitants was 50,000, it is evident that some pernicious influence was at work to retard the development of England's most important American province.[5-41] A drop in the rate of increase from 200 per cent during the thirteen years prior to 1662, to 25 per cent in the thirteen years following, is a clear index to the startling change brought about in the colony by the British trade regulations. These figures are the more significant in that there was no appreciable slackening of the stream of servants. It is probable that in the period from 1662 to 1675, which marked this estimated increase of 10,000 persons, fully 20,000 immigrants had come to the colony.[5-42] The patent rolls for 1674 alone give the names of 1931 headrights, and this year is by no means exceptional. No wonder Edward Randolph was surprised at the smallness of the population and wrote to the Board of Trade that it should be investigated why Virginia had not grown more, "considering what vast numbers of servants and others had been transported thither."[5-43] But Randolph failed to realize that it is not the volume of immigration but the number of people a country will support which in the end determines the size of the population. It was not enough to pour into the colony tens of thousands of poor settlers; opportunity had also to be afforded them for earning an adequate living. And this opportunity, because of the enforcement of the Navigation Acts and the consequent ruin of trade, they did not have in Virginia. Throughout the Restoration period not more than forty or fifty thousand people could exist upon the returns from the tobacco crop, and beyond that the population could hardly rise. If more poured in, they must of necessity live in misery and rags, or migrate to other colonies where more favorable conditions existed.