The retardation problem, for example, has become serious. Statistics indicate that from one third to one half of the children in the public schools of the United States fail to advance with the speed expected of them. Ten to 15 per cent. are retarded two years or more. Five to 8 per cent. do not come within three years of the state of development set as a standard. More than 10 per cent. of the $500,000,000 spent every year in this country for school instruction purposes is used for reteaching children what they already have been “taught” but have failed to learn. Many efforts toward reform have been fruitful but disappointing. The supposition that evils in existing systems could be completely cured by adopting new methods of instruction, altering promotion methods, giving increased attention to children’s health, and adoption of other innovations, was less effective, experiments have shown, than was generally anticipated by educators who put these theories into operation. These reforms were less successful than their authors expected they would be, for the reason that the reformers fell into the error of assuming that, under the right conditions, all children would be equally, or almost equally capable of making satisfactory progress. They failed to take into account the fact that there are more than two classes of school children and that they cannot be graded merely as “feeble-minded” and “normal.” There are all degrees of intelligence, ranging from idiocy on the one hand to genius on the other, and any efforts toward improvement of conditions must be applied with full recognition of such differences. There are wide differences among normal human beings in mental inheritance and these differences affect to a marked degree the capacity of men, women, and children to profit from instruction. Just as the Army had to allow for differences in mental capacity, so must the schools differentiate courses of study in such a way that each pupil will be allowed to study in a manner that is easy for him, whether that manner be rapid or slow. Dr. Lewis M. Terman, Professor of Education at Stanford University, in California, who writes with more authority than any other author on the application of psychological tests in schools, emphasizes the fact that little progress can be made toward the correction of present evils until we acquire a more scientific knowledge of the material with which we deal. This phase of the problem perhaps suggests the only practical way toward solution. Intelligence tests in schools and higher institutions have been given a wide range of application, but in virtually every instance the results have justified the claim of superiority for these tests over other methods of classifying students. In some instances positively startling developments have been noted.