THEY are afraid to say so in public, but many of the North’s big-city mayors groan in private that their biggest and most worrisome problem is the crime rate among Negroes. In 1,551 U.S. cities, according to the FBI tally for 1956, Negroes, making up 10% of the U.S. population, accounted for about 30% of all arrests, and 60% of the arrests for crimes involving violence or threat of bodily harm—murder,...