The real reason for the passage of these laws is not always clear. They may have been passed either to keep negroes out,[28] or to raise revenue for the government.[29] An analysis of the laws themselves seems to show that both of these purposes were constantly in mind.[30] When, however, they are taken in connection with matters which they themselves do not mention, namely, the predominance of the Quakers in the colonial Assembly together with the abhorrence which they felt for the slave-trade and later for slavery itself,[31] it becomes probable that the predominant motive was restriction.[32] It is also probable that while the obtaining of revenue was the obvious motive in many of these acts, yet revenue was so raised precisely because Pennsylvania desired to keep negroes out; that imported slaves were taxed largely for reasons similar to those which caused the Stuarts to tax colonial tobacco, and which lead modern governments to tax spirituous liquors and opium. It may be added that Pennsylvania always held, both in colonial times and afterwards, that England forced slavery upon her. That there was much justice in this complaint the failure of the earlier legislation goes far to sustain.[33] The negroes imported were brought sometimes in cargoes, more often a few at a time. They came mostly from the West Indies, many being purchased in Barbadoes, Jamaica, Antigua, and St. Christophers.[34] As a rule they were imported by the merchants of Philadelphia, and, being received in exchange for grain, flour, lumber, and staves, helped to make up the balance of trade between Philadelphia and the islands.[35] A few seem to have been obtained directly from Africa. When so brought, however, they were found to be unable to endure the winter cold in Pennsylvania, so that it was considered preferable to buy the second generation in the West Indies, after they had become acclimated.[36] Some were brought from other colonies on the mainland, particularly those to the south. At times Pennsylvania herself exported a few to other places.[37] The prices paid in the colony naturally fluctuated from time to time in accordance with supply and demand, and varied within certain limits according to the age and personal qualities of each negro. The usual price for an adult seems to have been somewhere near forty pounds.[38]