As the nation developed this slave labor became confined more and more to the raising of cotton, although sugar continued to be the chief crop in the West Indies and Louisiana, and rice on the southeast coast and tobacco in Virginia. This world importance of cotton brought an economic crisis: Rich land in America, adapted to slave methods of culture, was becoming limited, and must either be increased or slavery would die an economic death. On the other hand, beside the plantation hands, there had grown up a large class of Negro servants and laborers who were distributed both north and south. These laborers in particular came into competition with the white laborer and especially the new immigrants. This and other economic causes led to riots in Philadelphia, New York and Cincinnati and a growing conviction on the part of a newly enfranchised white workingmen that one great obstacle in America was slave labor, together with the necessarily low status of the freedmen. These economic reasons overthrew slavery.[58] After the legal disappearance of slavery its natural results remained in the mass of freedmen who had been trained in the necessary ignorance and inefficiency of slave labor. On such a foundation it was easy to build and emphasize race prejudice. On the other hand, however, there was still plenty of work for even the ignorant and careless working man, so that the Negro continued to raise cotton and the other great crops and to do throughout the country the work of the unskilled laborer and the servant. He continued to be the main laboring force of the South in industrial lines and began to invade the North. His full power as a labor reservoir was not seen until the transformation of the World War. In a few short months 500,000 black laborers came North to fill the void made by the stoppage of immigration and the rush of white working men into the munitions industry. This was simply a foretaste of what will continue to happen. The Negro still is the mightiest single group of labor force in the United States. As this labor grows more intelligent, self-conscious and efficient, it will turn to higher and higher grades of work and it will reinforce the workingman’s point of view.[59]