An early study by Charles H. Judd (1908) proved to be especially powerful in arguing against Thorndike’s theory. In Judd's study, fifth- and sixth-grade pupils practiced shooting darts at an underwater target Those who had been taught the principle of refraction were more adaptable to changes in the depth of the target than those who had not been taught the principle. Judd argued that transfer occurs through generalization. He contended that although generalization might not occur at the Iower levels of mental activity, it is typical of the kind of higher-order thinking the schools should cultivate.