Less promising was the broad substratum that sustained the prosperity of the colony. For fifty years indentured servants were coming in at a rate from a thousand to sixteen hundred a year. No doubt many an enterprising wight of the English or Irish laboring-class sold himself for a term into the tobacco-fields in order to come within reach of beckoning Opportunity; but we know, too, that the slums and alleys were raked for material to stock the plantations. Hard-hearted men sold dependent kinsfolk to serve in the colonies. Kidnappers smuggled over boys and girls gathered from the streets of London and Bristol. About 1670, no fewer than ten thousand persons were "spirited" from England in one year. The Government was slow to strike at the infamous traffic, for, as was urged in Parliament, "the plantations cannot be maintained without a considerable number of white servants." Dr. Johnson deemed the Americans "a race of convicts," who "ought to be content with anything we allow them short of hanging." In the first century of the colonies, gallows'-birds were often given the option of servitude in the "plantations." Some prayed to be hanged instead. In 1717 the British Government entered on the policy of penal transportation, and thenceforth discharged certain classes of felons upon the colonies until the Revolution made it necessary to shunt the muddy stream to Botany Bay. New England happily escaped these "seven-year passengers," because she would pay little for them and because she had no tobacco to serve as a profitable return cargo. It is estimated that between 1750 and 1770 twenty thousand British convicts were exported to Maryland alone, so that even the school-masters there were mostly of this stripe. The colonies bitterly resented such cargoes, but their self-protective measures were regularly disallowed by the selfish home government. American scholars are coming to accept the British estimate that about 50,000 convicts were marketed on this side the water.