Slavery is useful for early accumulation of capital, but it is too rigid for industrial development. Slaves had to be given crude non-breakable tools which held back the capitalist development of agriculture and industry. That explains the fact that the northern portions of the U.S.A. gained far more industrial benefits from slavery than the South, which actually had slave institutions on its soil; and ultimately the stage was reached during the American Civil War when the Northern capitalists fought to end slavery within the boundaries of the U.S.A. so that the country as a whole could advance to a higher level of capitalism.

p. 136. - How Europe Underdeveloped Africa (1972)